Snow boots always run larger than athletic shoes, and these will run about 1 size bigger than what your LO usually wears.  But throw on some thick socks and let them grow into these.  We have spent entire afternoons playing in Wisconsin snow (6"+) and my toddler's feet were warm and dry when we got back in the house.

My 7 year old son had this brand of boot as a toddler as well and quality for the price cannot be beat.