Depends in the kid. Dd is a puddle jumper and soaks her trousers so I try and get her to wear her all in one. Hers is red and made by Muddy Puddles - I bought it from a farm supplies shop it wax quite cheap but v durable. If you go for all in ones size up as much as possible and have them baggy as they grew out of them fast I'd the zip goes down between their legs
I found a good Regatta rainsuit online. Great for when you have to get out of the house regardless of weather. Great for playground- slides etc as water didn't get in at all and he was fine with layers underneath for warmth. I've got waterproof trousers from Jo Jo Maman as well but they're about as longwinded to put on as the suit and he still seems to get wet feet & ankles from all the puddles.