We do not recommend bicycle seats. We used one for a year and found that the bike was very unstable, when starting, stopping and getting on and off the bike (as it can make the bike top heavy when stationary). If the bike falls (which it nearly did with us), your child can fall some distance with the bike!
These pictures show a badly fitted example where the fixings make the seat sit high up. However, even where the seat sits lower (just above the wheel), it still means that your child can fall some distance if the bike falls over. Please consider using another method of transporting you child. (If you must use one, look for one that gives your child the most protection should the bike fall).