New and trying to solve some problems

Hi there

New to this and very confused!

We have 3 kids and would like to hire a motorhome next summer before jumping in and buying one. We have already come across some headaches!

have discovered that my other half (the driver) would need to sit a test to be able to drive a larger van, which is possible (he passed after 97) but am finding it hard to find information on larger vans that are under the 3500 limit for his license.

The biggest headache however is actually fitting the kids in safely.
When we would like to go we will have an 8 year old, a 6 year old and a 3 year old. This would mean they all have to be in car seats, the two oldest in booster type seats.
Does anyone know a way to do this? We *think* that due to the way booster seats works the two oldest would have to use the 3 point belts that are forward facing.... yet am really unsure if there is such a thing as a rear facing/lap belt compatible car seat for a then 3 year old.....

Ok.. am confusing myself too now. The trip would be around France/Spain if we can pull it off.

ANy advice would be appreciated as we are overwhelmed with problems at the moment and don't want to give up! and surely there are motorhome users with 3 kids under 12 somewhere out there!

Thanks!!!!!