Niecie, if you're only sending one then do it when the children arrive. Otherwise send a card for both stages, it could be up to a year between the children being placed and the adoption being finalised and that would be a long time to wait to congratulate them.
I sent a blacnk card to my cousin when she got her baby, and I just wrote my own message. But the card I got had a baby with a telephone on it. I'd seen others that would have been nice too. not impossible to find a suitable card.
Thats what we did in the end - I think we sent a congratulations one after court, which was 18mths down the line in their case. We couldn't send a new baby one as the children were 2 and 5yrs so sent the adoption one then as it was more appropriate.
I got one for a friend that said "Brand New Mummy and Daddy" (which wouldn't be any good for a second adopted child!). I know it takes ages before adoptions are finalised so have actually given it to them now they have a date for bringing him home (he's overseas).
Even though he'll be under 1, all the baby boy cards had prams or rattles and he's not that small!