Legal Question in Family Law in New Mexico

There is no concrete response to your question. If an adult left the two you describe at a public theater and picked them up after the show was over, there is probably no problem. There is no certain age at which a child may be left under such circumstances - it depends upon the maturity level of the child and the safety of the forum. Leaving them on the street would be child endangerment; leaving them at a crowded theater probably is not.