Once your first child is old enough he can go in to a foam booster seat - that is all they need at that age and they are actually safer than 'proper' structured car seats with 5 point harnesses in the event of an accident. I know that sounds silly but my DH is a registered car seat fitter and has done the training!! So the foam seats are only there to boost your child to a height so that the seatbelt fits, and need no anchor points. He will be fine in the back seats then.

And as for twins.... deal with that if it happens!!