I used one that sits behind you, Hamax Siesta, and attaches to a bracket the fixes to your seat tube. My lad seemed to be v comfy in it but it did bounce about quite a bit. Top peak and Hamax both do one that fits to a sort of pannier rack and has some of it’s own susspension, so i reckon off road these would be better.
Then there are the ones that sit infront of you (wee-ride or something) on the top tube, good for talking to the little un but aparently these can come up quite small so it depends on how big you lad is.
My lad is now on a saddle that sits on the top tube and he holds onto the handle bars, but he is nearly three now so I wouldn’t go for that just yet if yours is only 18 month.
J E James, Halfords, Evans, wiggle and CRC all stock different ones.