I always like to bring an amount for our food and drinks on a gift card. Like others said, there is no way to currently store a gift card on the app (it has to be entered each time) outside of the Apple or Google Wallet workaround. When we went in April, I had the gift card info on my notes app that I could cut and paste if I was doing a mobile order. But most of the time, I preferred to just use my physical card and walk up to counters.
The cut and paste method may be too much for a kid, depending on the age. Physical cards may be best. I REALLY wish we could pay with our MB+s! That would be amazing.