I've just asked my dd (teen with diabetes) for her input and got the usual mouthful but hey she is a teenager.
Anyway I'd ask the parents if I were you. Different diabetics are on different regimes - some have to snack, some have to inject insulin according to the number of carbohydrates in the food, some are fussy eaters and some can have other food requirements. You should also ask what to do if one of the kids isn't well - if they know they are having a hypo (low blood sugar) and do they do their own injections, blood tests, etc.
To DJM, adults can have Type 1 or Type 2 diabetes. Type 1 is what kids get and is always controlled by a combination of diet and insulin. Type 2 can be controlled by diet and/or diet and tablet or diet and insulin. So if a kid has diabetes, there will be insulin injections and blood testing involved.
My dd has pizza, crisps, cheese straws and Muller Rice for treats. She is also a fussy eater so I generally send food with her. Fruit (apart from bananas) sends her blood sugar too high so fruit is saved for when her blood sugar is low or when she is going to inject more insulin or do some vigorous exercise.
Also too much artificial sweetener can cause diarrhoea in some people ... maybe not the best idea for a picnic. Honey has much the same glucose content as sugar sadly.
Sorry to go on but it's a complex thing for Type 1s. Hope this helps. Please don't exclude the kids with diabetes - my dd lost out several times including school trips which made both of us sad.
PS Frozen bananas are great.