I was born in Peru, I am an American citizen now, my friends asked me to take their kids to Mexico and then to bring them back, this kids are born in usa, but their parents are illegal, I want to know if this is illegal to do.

If you can
You would need to get any required documents and some sort of temporary guardianship papers so that they can not be confused as kidnapped
I would think

I was born in Peru, I am an American citizen now, my friends asked me to take their kids to Mexico and then to bring them back, this kids are born in usa, but their parents are illegal, I want to know if this is illegal to do.

Traveling with minors that are not your children, you need proper documents signed by legal parents.

On personal note, I would not recommend doing this because in the documents, the parents need to sign and give all details about contact address, phone numbers, travel expenses, etc. This will send red flag to immigration and the parents are likely to be tracked down soon.