For my DS's birthday party we are thinking of Booking him a party at a farm, obviously we'd pay for the children and each child can bring one adult free of charge but additional adults are £3.
Wibu to mention this on the invites and ask parents to pay for themselves if they are both coming?
There's also all the grandparents who would want to go (he has 8 including great grandparents so it would work out as quite an extra stamp on the bill)