Puffin Books is the long-standing childrens imprint of Penguin Books and was formed in 1940.
Piccolo Books is the children's imprint of Pan Macmillan.
Armada Books was set up by Gordon Landsborough in 1962 as a paperback imprint of Mayfair Books Ltd, focussing exclusively on books for children to buy with the pocket money. Collins bought it in 1966 as an imprint to publish books for 10-15 year olds under their Fontana Books paperback arm. Armada ceased in 1995 but I will always love it because it published The Three Investigators.
Beaver was the children's imprint of Hamlyn which is now part of the Octopus Publishing Group, owned by Hachette Livre.
Fontana was the paperback imprint of William Collins & Sons and is now part of Harper Collins.
Magnet Books was the children's imprint of Methuen