Wetsuit Centre is the UK's largest stockiest of children's wetsuits both for summer and the winter months. If you are looking to buy a kids wetsuit you should consider which months of the year they will be using the suit.
So from the months of May to October when the water temprature is from 17c to 20c a 3/2 full suit would be the best option as this means the body of the suit is 3mm of neoprene and the arms are 2mm . The neoprene traps the a thin layer of water against the body which helps to maintain body heat to a constant level. This means they can stay in the water for longer and more importantly stay warm and enjoy the sea.
If they are looking to only use the suit in the months of August then aa short arm and leg wetsuit otherwise known as a shortie. They are usually made in 2mm of neoprene as the water is warmer and thus having no arms or legs in the wetsuit is no problem at all. As for the water temprature it should be around 20c+ for most comfortable use of the suit.
For winter months from November to April where the water temperature is between 5c and 12c a 5mm neoprene wetsuit is the best option. The suits are 5mm on the body and either 4mm or 3mm on the arms. This keeps the body warmer in extreme winter waters. Some children also wear a 5mm wetsuit in the summer if the feel the cold.