I have a trailer I gto from
eBay for £60, which I use three days a week for dropping my son off at nursery. I've heard people say that on the flat you should hardly notice a trailer, but I've not found that to be the case with ours - there's a definite drag. Of course, that might be due to it being a bargain basement trailer rather than a better made model. Anyway my son (also 3) loves going it in. I do find it a bit difficult to hear what he's saying to me from back there though.
I think if you go a bit more expensive, you'll get a lighter trailer (ours is something like 19kg unladen..
), but for short journeys I've not found it a problem (although it does take a while to go uphill!)