I have the annual pass it works batter for us. Every other week, we go out. It is cheaper and we visit chessington and Aquarium.
From Merlin website: Children over the age of 3 will need their own Merlin Pass or ticket to enter our attractions. Children under the age of 3* go free! *At LEGOLAND Windsor Resort, children under 90cm go free. However, under 3s will still be free when accompanied by a Merlin Passholder.
Children under 3 are free with a Merlin pass. Children 3 and up will need their own pass.