Child 1: dairy and soya free - a lot for commercial bread contains soya flour. Make your life easier by buying all the bread products as soya free.
Morrisons and Sainsbury's are poor for sliced bread as well except their bakery breads contain soya flour. Tesco, Coop, Aldi and Waitrose own bread is usually soya free if wholemeal mixed results for white bread.
Bread products like wraps, bagels, pittas, baguettes and so on are usually soya free.
I would be inclined to pick something like wraps or pittas where it's less noticeable which are regular and which are wheat free.
Buy a marg that is dairy free and use that for everyone - there are quite a few on the market now including pure, vitalite, all of the flora range, stork baking block is good for baking with if you fancy having a go of baking a cake
You should be ok with free from cakes that are dairy, soya and gluten free. I can't recommend any as my daughter is egg and tapioca free but ok with wheat /gluten so we have give this miss as egg and tapioca are used to replace wheat /gluten
We do homemade sausages rolls quite often as regular packet puff pastry is dairy and soya free. We pick our own sausages to ensure they are also safe (other allergens) and are usually tastier than packed sausage rolls. There are gluten free puff pastry rolls available too.
I would check with the parents to see if they have any preferred products. I pass on 3 or 4 different dairy, soya and egg free cakes commercially available cakes when my daughter goes to parties as I know these are readily available in our local supermarkets and that she will and can eat them! My daughter isn't a fan of chocolate products for example so I tend to steer them towards vanilla or lemon sponge cakes rather than chocolate for example.
Doing their lunches first is a good idea, this is what I recommend whenever anyone is cooking on mass for my daughter as it's the easiest way to control for cross contamination - I'd do child 2 before child 1s also.
I would pop a note on or in the lunchbox too so some one can check through what should be in there before giving it to the child and have a copy of all the ingredients at hand too. No matter how much a trust a person to make food for my daughter I check every time - ingredients can change from the last time I bought something and it's easy to miss something. My daughter was given something in a packed lunch on a school trip by the chef that had cooked for her every day for nearly a year. Now her packed lunch is checked before it leaves school and before it is given to her