Should it be mandatory to pay for a bounce house for kids??

So I have a family 4th of July party coming up this weekend,and my cousins wife is the one who put it all together,it is being hosted on my Nana's land. His wife told everybody that it was a mandatory $20 per person,not per household, to pay for food and the bounce house for the kids. Now,I don't mind contributing to food,and my husband and I are bringing fireworks to contribute to the display,but we do not have children. Personally,I don't think its fair for us to have to pay $40 to entertain other peoples children( I also am trying to stay on a budget to pay off medical bills so I can afford a procedure to diagnose my endometriosis) Understand,most of the kids/family that are going to be there is her side of the family,even though my nana is hosting it on her land and using her house. Do yall think I'm overreacting? Should it be mandatory for all to pay that much,or only the parents/adults who are bringing children should pay extra to contribute to the bouncy house??