Limited invites

So I am looking into booking an indoor play area for my daughter’s birthday. There is a limit of 30 adults and 20 kids (but can have additional kids for $12 extra per kid). Because of this, I am only inviting friends and families who have children. Both parents are invited of course however how do I let in the most polite way that only parents and not additional family members are invited due to this limit? For example, my husband’s cousins have “adult” children (both in their early 20’s) but they cannot come as they would take up limited space that is reserved for other parents—- I already know that my husband’s aunt (through marriage that we rarely see as his aunt and uncle are separated) is going to make a scene about not being invited.

Thank you!