In reply to Bobling:
Can't comment on bike storage except to suggest you also invest in some decent security.
Regarding transporting them, though, you have a couple of choices. Easiest would be to get a rear-mounted rack for the adult bikes and put the kids bikes in the boot. You may need a lighting board if you're obscuring the lights, but most cars with 2 bikes can get away with it - take wheels off and put them inside if you need to.
Otherwise, your best option is a tow bar rack, as mentioned above, but they are expensive to buy and you'll need to factor in the cost of a tow bar and installation, including electrics. My neighbour has the Thule one and loves it.
Most portable between cars are roof bar carriers - I've got Whispbars, which were the cheapest by far for fitting kits. When I priced them up, Thule and Whispbar are about the same price for a single car, but in order to get them to fit both our cars, the Thule ones were around £100 more expensive. I use them with the Thule fork-mount carriers.
In any case, have a look here for what you need - they often have bargains where people have returned items:
https://www.roofbox.co.uk/
Edit - thinking about it, by the time you've bought bars and 4 carriers, there probably won't be much difference in price between buying a tow bar rack - it'll come down to how often you use it and when you're planning on changing cars.
I went for the roof bar ones mainly because I already had roof bars for carting surf boards and kayaks around. These days, I am limited due to the difficulty of having a tow bar fitted to a company car.
Post edited at 10:35