I’m not up to date with books for this age so sorry I have no recommendations. But just wanted to say that your children will probably accept this far more easily than any of the adults in your friends lives.
My 12 yo son plays in a sports team , all the parents know each other. Last year I saw this random toddler running around and all the 11 and 12 yo boys chasing her . Asked Ds who she was “ That’s Jane’s [coach] daughter “.
I knew Jane didn’t have kids so like a fool I said “ don’t you mean her Niece, her sister Mary’s DD? “.
“No” said my son rolling his eyes , “she just adopted her”. He didn’t say the word stupid at the end but he implied it.
And that was that . Not one child in the team batted an eyelid . She was completely accepted from the very first day. The only talk I heard from the parents was “ how lovely “ and everyone bought gifts.
I never heard anyone ask anything about the circumstances . None of these people know that we have adopted children so they weren’t being polite in front of me.
Even though the boys were old enough to ask difficult questions, none of them did. They just enjoy teaching toddler to play their sport.
Most children just accept these things. Yours are too young to even ask much. They will just be pleased to have playmates .
You can just say eg
James and Emma are getting a little girl. Her name is Sophie and she is 3. They will be her mum and dad .
They might ask why they are not growing a baby in her tummy. You can either say that they tried but it didn’t work ( assuming you have friends permission to say that ) or just that they decided that they wanted to be parents to Sophie and not have a tiny baby.
Your 5yo might ask if she has other parents. You can say that she did when she was born / until she was one but they couldn’t look after her. So she went to live with foster carers, who are a bit like an aunt and uncle. They look after you but they are not your mum and dad.
So social workers had to find a new family and James and Emma will now be her mum and dad forever.
Your children will probably be more interested in what Sophie looks like and if she plays football / likes cars than much background detail. Children are very accepting .
Personally I wouldn’t discuss it with your children until your friends have the child/ ren placed . Because your children will want to meet the new child straight away and that’s not going to happen in the first few weeks after they arrive.
Also you will be able to show them a photo of the new parents and child together and photos of the child’s room. That will make it more real for them.
They can also help choose a small toy for “Sophie “. Don’t give clothes, unless Sophie is a baby.
(BTW I’m assuming that your friends are an opposite sex couple adopting one child just for the sake of simplicity , but I know they might be an opposite sex couple adopting siblings )