I think that these are totally cute, and very easy to make.
First - take an empty cardboard box from the recycling and cut out a snowman shape.
Next - provide your toddler with glue and encourage them to liberally spread it all over the snowman shape.
Provide them with cotton wool balls and show them how to tease out the ball shapes to make "clouds", then get them to stick the cotton wool all over the snowman.
For the next stage it depends on the development of your child. If they are happy to do it, then just provide them with some coloured paper scraps and encourage them to cut out appropriate pieces for hat, nose, eyes, buttons etc. If they are still... well... anything like Bug, then you need to cut out the shapes and direct them exactly where to stick them on (she really wanted to stick the nose on to her own nose... and to stick everything else to the table!).
Et voila! Gorgeous snowmen to decorate your home.