Restaurant Reviews & Suggestions - Where to eat?

Going to Buckingham Palace and need somewhere to eat after.

Criteria:

1. Not one of those shit expensive places you get (or used to) near Trafalgar Square
   
2. Accessible with two small kids - so bus or not an insaine walk
   
3. Small kid friendly - both price and food options. Eldest will eat absolutely anything. Youngest is more fussy, but still OK. It's more speed and the carnage they can bring.
   
4. Open today
   
5. Oh yeah gluten free options
   
   

Chinatown feels like a bit much, both finding somewhere and the journey. That massive mall in Victoria with Nandos, Wagamama, etc.? Was hoping for something more exciting, but we could play our way through St. James and get to the shopping centre.

Any bright ideas appreciated.

Cheers.