I think trailers are the best way to transport kids, certainly if you have two. It is hard to imagine, but your kids will also grow out of them, and it pays to get one with a bit more headroom, particularly if your kids are tall for their age, so you get another half year or even year out of the trailer. .About five years is the max for many children and many trailers, but every half year extra is a real bonus, because at five or so they will often not be able to ride their own bikes in traffic. For the next phase we bought a second hand Thorn childback tandem, which was great, but at four many children will still fall asleep on the back of the tandem....
The other thing to consider is replacing the cheap, stiff and slow tyres that nearly all of these trailers come with. Fit the widest Schwalbe Big Apples that you can squeeze in, and your kids will love the extra comfort (important, especially when they are very young), and you will appreciate the reduced effort.