Not need if no skiing or playing sleigh with kids. Unless he is not used to cold weather, then can get one. Otherwise, we wear our usual pants, or put on pantyhose under the pants, then it's warm enough. Most important is upper body, neck, mouth, head.
But my children wear the one that's similar to PUIPUI2000's picture, and used it for skiing in two years, no problem, because I let her wear fleece pantyhose and long socks. So this will do if only mild skiing. Otherwise, I still think formal ski pants are much preferred, from taobao, I bought kids one only cost RMB79, it's formal and water proof, and thick. Very good!
From my experience, as long as the pants are waterproof and thick, it's fine. Even I ski with a down jacket without waterproof, I was still perfectly fine because I seldom fell off, even I fell off, the snow is not watery that would immediately permeate into the clothes underneath. For non-professional skiers, the usual ones are no problem. Don't worry.
But if you really want to be guaranteed, buy those from professional skiing pants in HK that cost you a fortunate ...then you are hassle free!!