YANBU. They are being very U.
The snack is a bonus in a FREE group (as you say).
People tend to forget that food has to be paid for, and that someone has to prepare, serve and wash-up afterwards, as well as wiping down tables and cleaning floors - we all know how much mess a small child can make even with a single rich tea biscuit. And tea/coffee for the mams? Some poor soul has to faff about boiling kettles, making sure that there is sufficient tea, coffee, milk, sugar, sweetener etc. And then there is sure to be someone whose child can't won't eat particular foodstuffs, the kid with allergies - once this starts it can snowball if the group is as entitled as some of these mothers seem to be,
It also causes safety issues when there is hot liquid around toddlers, and environmental health ones around the providing and serving of food, especially food that requires even minimal preparation, such as sandwiches - preparation areas have to be regularly inspected, etc and food hygiene certificates obtained. It's a faff, can be expensive and is time-consuming.
If these mothers think their children need a lunch, then they should provide a packed lunch for them - or perhaps offer to provide food for the group (they could take turns) and make sure that they take their rubbish away with them. They could take themselves a flask.
I'm totally on your side here OP. It is a free group - be grateful for it, as many areas don't have free anything. If a child won't eat fruit (which is pretty unusual - maybe the mother should look at what she is offering at home), then they have their rice cake or whatever.
The foods on offer seem to be the ones least likely to cause stomach upsets, allergies, or be eaten from greed rather than genuine hunger. If you are able, perhaps remind them that this isn't a party - it is a playgroup and if they want more they should go somewhere else and pay for the extras (diplomatically, obviously - not the way I have put it
).
The likelihood is that the group facility will be ended if they demand too much and it starts to cost a lot of money.