Ds1 (age 7) has the 3 in 1 regatta and ds2 currently has the Regatta Reversible (he is 4 and about to start reception)
The 3 in 1 is great if it is warmer and pours with rain, very sturdy, well made and paired with waterproof trousers has kept ds1 dry walking to school in torrential rain.
Ds2 had the reversible one from age 3 and it does cover his bottom, just. But if it is warm you can't detach the fleece, also if they take it off and the sleeves get turned the wrong way, teachers tend to help them back into the coat with the fleece the right side out.
If you walk to school, then also look at waterproof trousers that have a zip at the bottom of the leg meaning you can tuck their school trouser into the wellie then put the waterproof trousers over the top. I have had both togz (expensive) but good for ds2 and Peter Storm ones.