The thing about airports is that every day, people from around the world with many different languages are passing through.
In the US, especially in Texas (where you mentioned) and Arizona, they're likely to speak some Spanish. However, even if the staff don't, they're used to dealing with people every day who don't speak English - they may speak Russian, Mandarin, French or whatever else comes up.
As long as they have their travel documents and ID, showing this to someone with a questioning expression is pretty much guaranteed to get someone to help you in the airport, from staff to other passengers.
Remind them also, that in the airports, everything is well signposted in international symbols for this very reason. Departure signs, passport stamp signs, boards with flight numbers and destinations. Aside from the check in and if you go through passport control, you hardly even need to communicate with anyone, just follow the signs.
However, even if they can't find staff, asking passengers around if they speak Spanish - in the US I found many people tended to even have basic Spanish (especially in certain States, like Texas and Florida) and if not, people are often happy to try and help regardless.
Hope they enjoy their flight and the fun experience of travelling!