If my DS was one of your invitees, I'd have no problem sending him with a packed lunch. You are taking them out for pretty much the whole day, are paying for the activity as well as covering birthday tea (& transport?) and I would think nothing of it if you asked me to supply a lunch.
However, only letting them know the day before that this is required isn't great, so I'd make sure explain its down to logistics as your helper monkey has taken ill.
I don't drive and sometimes it doesn't occur to friends who drive that if we are doing X, I can't reasonably manage to lug a,b&c on the bus, even though they can just chuck it in the car boot .
As for making up lunches yourself?
There have been loads of sensible suggestions
One bag each with a sandwich, a juice carton, fruit, crisps.... Include a variety of 'standard' fillings & flavours and tell them to trade any they don't fancy.
I love the sandwich loaf idea someone mentioned.
Hot dogs in a flask of hot water stay warm for ages.
Individual packed lunches could be packed together and your boys in charge of those until you get to train station, then distributed.
Make sure there's an extra large bar of 
in yours, and a 
waiting for when you get home!
Hope you all have a wonderful day!!