Really? When? When what is now Mexico and what is now California belonged to Spain? So what the kid is missing and should have said is: "This state and Mexico used to all be part of Spain."
"We can't make it illegal for them to be here."
Yes "we" can, just as Mexico made it "illegal" for Spain to continue laying claim to Mexico. America did the same with the land now in what is called California. Mexico-independent-of-Spain did not preceed California did it? Am I mistaken? (embarassed that this multi-generation California native with relatives who still reside on Spanish land grant property that was given to them by the King of Spain isn't as clear on this as she should be!)
The official year of the independence of Mexico from Spain was 1821. California was ceded to the United States in the Treaty of Guadalupe Hildago (ending the Mexican War) on 2 Feb 1848, so there's a few years when California was part of Mexico rather than Spain.