I would doubt that every parent sent in $15 per child, therefore, there is a shortage of funds. There is no way you can have a party with any type of game or craft for $5 per child unless games, arts, crafts, food, supplies are donated by parents.
In our elementary school, there is a sign up sheet for what is needed for the parties a few weeks before the scheduled parties and families donate the items. Some donate more, some donate nothing.
I would go help with the party because it is a lot of work for 1-2 room moms to organize and create a fun party for everyone, especially when so many parents just don't volunteer or donate.
In my day as President of booster club and more, it amazed me how many people did not donate AFTER they voted to do something which benefited their child and then the booster club was obligated to pay for everyone. It sucks but some people just don't care and it leaves the parents who do care out more money, supplies, etc.
I'd give this mom the benefit of the doubt because I have more than once paid out of my pocket for those who did not donate when they said they would.