I think it’s because she was being cagey about how she answered the questions. People specifically asked if the invite or parents had actually said that the partygoers parent(s) need to stay with the child, or if she/the child’s father were the ones who were wanting to stay - she just kept replying saying the children were reception age, 4-6 (which neither answers the question and raises an additional one about what 6 year olds are in reception). She was then asked how old HER child specifically is, and she kept saying the group were reception age, 4-6. Again, not answering the question asked.
She still hasn’t said that the invite or parents have specifically said that they have to stay, although she has said the parents expect them to stay, but later mentions that she wouldn’t want to drop and leave.
I suspect that the invite doesn’t say anything about HAVING to stay, but says if they choose to stay it’s £15 per adult. It may well be that the parents of the birthday child already have enough adults (eg 2 parents, 2 sets of grandparents, aunts & uncles) that are either included in the party package cost or they’re paying for - so they don’t want to pay for additional parents to stay when it’s unnecessary if they feel they have enough already. If this is the case, I think the OP is being unreasonable. If the party hosts don’t have enough adults to supervise and/or they specifically said on the invites that all children must have an adult stay with them and the cost for this will be £15 per person, please send this money to X bank details by Y date then the OP is not unreasonable, and the hosts should have booked something they could afford.
I really do think though, based on the vague answers and not wanting to actually answer the questions asked, the hosts haven’t actually said the words that all children must be accompanied.