Is this an unreasonable bday invite request?

[Anonymous]

Anonymous wrote:"I'm sorry but we cannot add any additional people to the birthday party package. FunZone does have open play at the same time if your husband and son want to take advantage of that during the party time. Feel free to drop off your child too if you'd rather. We'll be in an enclosed space and have plenty of supervision"

just make sure you mention that there will be a fee for the other kid. your post makes it sound like the open play is free.