I would avoid pre-purchasing tickets to include them in the invitation. If you include tix already paid for, the child may end up not going for whatever reason & that's money wasted as the parent may not return the ticket & just use it another time for themselves later.
I do think it's unfair to not invite siblings. I know that if I were a child & my sister was invited to Legoland & I wasn't I would be extremely disappointed and hurt that I was dis-included.
If you're insistent on that, and I do understand the reason behind that but again, consider the feelings of the siblings and either, as a suggestion, try to invite those without siblings or I would say somewhere in the invitation which child is invited and that you're only purchasing tickets for the children but also state that parents are welcome to attend with their children but please note that parents are asked to purchase their tickets and any for siblings that would like to come along as well.
So I guess you could say something like "Tommy is invited to son's birthday party at legoland...Tommy's ticket will be comped by us but we do ask that any parent and/or sibling that wishes to attend to please note that their ticket will not be included and will need to be purchased separately."
Hope this helps, good luck & have a great time!