I think a high tea party for the girls sounds fabulous. If that is the case I would clearly state on the invite the start time and finish time (I had a horrid experience of parents of twin boys not picking them up until 3 hours after the party had finished) and also that parents are welcome to stay if they choose. Although personally I'd rather they left if space is limited.
Good idea hun - definitely will do that!! Space is limited but if they are happy to hang out while we run activities for the girls then I am happy for them to stay!