S.S.
My suggestion is invite them, even if they can not skate they can still come and have fun and have cake and all that stuff, maybe you could have a particular job for them or something such as holding a limbo stick for limbo??? I don't know just a suggestion.
No, sinding invitations on the 19th would be fine for a party one the 24th. I used to send them before 1 week but found I get a better response sending it about a week before, may they get lost if it is too early or something!!