Personally I would still put a layer over the leggings like a moderate thickness pair of jeans.
My DD is one and at this time of the year she wears a bodysuit (babygro thingy), a long or short sleeve top, hoody or cardigan, a hat if she has no adequate hood and jeans or trousers with leggings or tights underneath, depending on how thick the trousers are. If I don't put any tights or leggings underneath then she wears thick knee high socks. I always double layer her socks or put a pair over tights as she isn't wearing shoe's yet, but still always wrap her bottom half in a blanket whether in her pushchair or carrier.
She is learning to walk now so the top layers won't change but thick warm shoe's will mean I don't need to double layer her socks.
I just think it is too cold for leggings alone right now but I can't stand even a little cold. Plus I don't have older children and maybe this makes a difference.