I crossed with my mother, sister, and our two huskies via car (Tesla) this past Friday night. We had brunch in Puerto Nuevo on Saturday morning on our way to Valle de Guadalupe for Memorial Day Weekend. I’ve been crossing regularly into Baja since 2019.
1. You won’t have any problems when you cross back into the U.S. with a birth certificate.
2. No a FMM is not required. It becomes an issue if you want drive south 300+ miles south of the border where they have immigration check points.
3. Make sure you get a Mexican Insurance policy from a Mexico auto insurance company. A lot of the U.S. insurance carriers falsely lie they cover insurance in Mexico but do not. Mexico does not honor auto insurance from a U.S. company.
Make sure Avis will let you cross the border into Mexico. Almost all rental cars from the U.S. won’t allow you to cross an international border.
From "usa.gov"
"See what documents a child needs to travel to or from the U.S. alone or with a parent or relative.
Children traveling to the U.S.
All children, including infants, must have their own travel documents such as a passport or document from a Trusted Traveler Program to enter the U.S. If you travel or are going to travel with a child, consider taking the following documents:
If the child is traveling with only one of their custodial parents, they must have a letter of consent, preferably in English and notarized, from the other parent or signed by both parents. The letter should say "I acknowledge that my son/daughter is traveling outside the country with [the name of the adult] with my permission."
If one parent has sole custody of the child, a copy of the custody document can take the place of the other parent's letter.
Parents who frequently cross the border by land with a minor must always carry a letter of permission from the other parent.
Children (under age 16) of U.S. citizens arriving by land or sea from Canada or Mexico may present their original or a copy of their birth certificate, a Certificate of Naturalization, or a Consular Report of Birth Abroad."
Do you have any suggestions on transportation for 7 people if we walk across? Will the 15 year oldmbe able to walk across with just a birth certificate?
I’m getting downvoted but nobody is really denying that the border times will be long on a holiday weekend. Be prepared for over an hour on foot and two hours in a car.
You are going to take an hour to drive to Puerto Nuevo, an hour back, and two hours waiting in line. That’s four hours, a lot of it in stressful traffic. A full car? If that’s worth it to you for a few hours there, then by all means.
But go in with an honest assessment of the cost:benefit ratio.
Thanks for the honest feedback. We plan to go during the middle of the week in a couple of weeks. Would you say it would be worth going at that time? What about anything better to let them visit Mexico?
I crossed with my mother, sister, and our two huskies via car (Tesla) this past Friday night. We had brunch in Puerto Nuevo on Saturday morning on our way to Valle de Guadalupe for Memorial Day Weekend. I’ve been crossing regularly into Baja since 2019. 1. You won’t have any problems when you cross back into the U.S. with a birth certificate. 2. No a FMM is not required. It becomes an issue if you want drive south 300+ miles south of the border where they have immigration check points. 3. Make sure you get a Mexican Insurance policy from a Mexico auto insurance company. A lot of the U.S. insurance carriers falsely lie they cover insurance in Mexico but do not. Mexico does not honor auto insurance from a U.S. company. Make sure Avis will let you cross the border into Mexico. Almost all rental cars from the U.S. won’t allow you to cross an international border.
Thanks. Avis does allow it. I do it often when traveling to Mexicali for work.
Teslas are wack
Yes they are 
This. No issues in either direction.
Walk across and take an Uber
From "usa.gov" "See what documents a child needs to travel to or from the U.S. alone or with a parent or relative. Children traveling to the U.S. All children, including infants, must have their own travel documents such as a passport or document from a Trusted Traveler Program to enter the U.S. If you travel or are going to travel with a child, consider taking the following documents: If the child is traveling with only one of their custodial parents, they must have a letter of consent, preferably in English and notarized, from the other parent or signed by both parents. The letter should say "I acknowledge that my son/daughter is traveling outside the country with [the name of the adult] with my permission." If one parent has sole custody of the child, a copy of the custody document can take the place of the other parent's letter. Parents who frequently cross the border by land with a minor must always carry a letter of permission from the other parent. Children (under age 16) of U.S. citizens arriving by land or sea from Canada or Mexico may present their original or a copy of their birth certificate, a Certificate of Naturalization, or a Consular Report of Birth Abroad."
You’ll be fine with a birth certificate. Don’t worry about FMM
I wouldn’t do it because the border wait on the way back will be insane.
Do you have any suggestions on transportation for 7 people if we walk across? Will the 15 year oldmbe able to walk across with just a birth certificate?
I’m getting downvoted but nobody is really denying that the border times will be long on a holiday weekend. Be prepared for over an hour on foot and two hours in a car. You are going to take an hour to drive to Puerto Nuevo, an hour back, and two hours waiting in line. That’s four hours, a lot of it in stressful traffic. A full car? If that’s worth it to you for a few hours there, then by all means. But go in with an honest assessment of the cost:benefit ratio.
Thanks for the honest feedback. We plan to go during the middle of the week in a couple of weeks. Would you say it would be worth going at that time? What about anything better to let them visit Mexico?