Hi Gear
Podsuit Kids Waterproof Jacket

I was really pleased with this rain suit. It's light so my little boy can run around as usual. The elasticated waist means the trouser part doesn't keep falling down! It's also very good value.

Got one of these for my little boy about a year ago (he was one and a half at the time), and had plenty of room to grow - given he is now the size of a 4 year old!

It has no thermal protection but spacious enough to go over a fleece/coat. Relatively easy to put on too - a major bonus for anyone who has stroppy offspring.