You're not going to find many (if any) restaurants in Austin or even Travis County that have that capacity that can do a dine-in for that many kids right now.
You're talking maybe something like Golden Corral - or another large buffet - or literally renting out a place.
Your best bet, given the current situation, is to get a box lunch deal from a sandwich place like Jason's Deli.
Remember that statewide, restaurants are limited to 50% capacity. A typical dining room at normal occupancy seats ~100-150 folks, depending upon restaurant size. Some bigger ones will seat more, of course. You're asking an average restaurant to devote an hour or more of their entire dinner service to only feeding your band.
