The double beds are great for a couple (and those with a small child) and are recommended if you're travelling with a partner
I don't think BA crew like having kids on the floor, but the last row provides a great space for you to put a blanket down and pen in a little one during regular flight (obviously you'd have to pick him up if the seatbelt sign comes on).
Are there any formal rules that would mean you couldn't do this?