You can hire my roof bars if you want, and tie a couple of passengers on with some bungee straps.
How about a roof box in case it rains - got to be considerate, you know...
I used to have one of those rearward facing seat conversions in a Citroen estate years ago when the kids where small. One of those and a towbar and a trailer job sorted!
Though I think my earlier suggestion of hiring 2 small cars would be the cheaper option