Desperately trying to find a book I read as a 10 year old in the early 90's. The book may have been published much earlier. The story still haunts me. It features a boy and a girl in a post-apocalyptic city (in the UK?). Their parents are missing. The boy finds a nest of eggs which hatch into lizards. The largest blue lizard bites the boy and girl on their necks and their faces are disfigured. Teaming up with an elderly homeless woman (who smokes cigarettes she pushes around in a shopping cart), they take the lizards into the countryside.
At the end, the old woman feeds herself to the lizards. It is also revealed that the lizards maybe dragons(?).
The children then found a new society.
Please help. This is driving me crazy.