I second The Suitcase Kid (Jacqueline Wilson) for 9-13. I read it as a teenager and it was my only experience of divorce and blended families until I entered this one. I recently gave a copy to my step daughter and she hoovered it up. Now she’s on her third JW in as many weeks!
It does handle lots of issues surrounding divorce. I re-read it after she’d finished it. It only took me about an hour and a half. It was a bit of an eye opener to see divorce through the eyes of a pre-teen girl.
But the step mum is nice!
Not about step mums but Two Homes by Claire Masurel is very good for much younger children.
For somewhere in between there’s a nice book called The Memory String by Eve Bunting. The mother in this story is dead but it’s about a young girl finding room in her life, her heart and on her necklace of memories for a new mother-figure in her life.
The issue with many fictional step mothers is that the mother has been killed off! It’s just “cleaner” that way because let’s be honest, nobody wants to read about our lives!
“And then Sue realised with horror, at the bottom of her laundry hamper were a pair of children’s underpants she did not recognise. Should these have been returned in The Bag at The Handover? She nervously broke the news to Pete. His expression twisted in agony. Together, they sat down, hands clasped, faces pinched, and opened Pete’s email. There it was. The email they had been dreading, glaring back at them from the white glow of Pete’s laptop screen. “Subject: UNDERPANTS. Sue broke down in tears. If only she’d stayed at home on Saturday and tended to the laundry. Alpaca Land wasn’t even very good. And the rides had cost extra….”