Sorry age between 3 and 6
Jonathan Stroud's award winning Bartimaeus Trilogy (in 4 parts with prequel) is younger - probably around 8 or 10, and is one of those children's series that there's a good chance parents will enjoy too. More alternate history and fantasy than sci-fi though. There's an adult layer in things, and a deliciously acerbic and sarcastic main character, so a good series for reading to them. I think I would have bought the rest of the series even if the kids hadn't enjoyed the first! Hugely recommended.
His other series, Lockwood, is similar but perhaps less for the adults though there's similar humour. They're a group of kids who are ghost hunters. So more squarely in the Harry Potter vein.
Younger still and I draw a blank, sorry - it's been a good while since our kids were in that age bracket. If I do remember anything suitable I'll update.
i pretty much enjoy any book that Stross publishes, including the merchant princes series, but things do get rather grim. (now kids, what do you think would _really_ happen if a superpower discovered a doorway into narnia? ...)