Not familiar with the Penguin reader series so hoping these are around the right level! From googling it looks to be around book band purple/ gold? Is that right?
Rabbit and Bear - easy books, lots of pictures and not many words on a page but each book is about a hundred pages! Quite funny.
Isadora Moon/ Mirabelle series (both by the same author, there must be over 20 of these now) - chapter books (just over 100 pages,), pictures on most pages but definitely a step up from Rabbit and Bear. Great stories about a half vampire, half fairy doing ordinary things like going to school/ the dentist/ on holiday etc. Think this is most likely to be the right level. These are very popular books so the library / Vinted should have loads.
Kitty and the … book series. Same level as the Isadora Moon. Kitty is a super-hero in training with cat like superpowers! She and her cats go on little sweet adventures.
Willow Wildthing series. Harder than Kitty/ Isadora Moon. Girl goes on adventures with her friends in the wilderness area of her town. These were really enjoyed.
We have just started Little Legends, about characters from fairy stories, Jack, Little Red Riding Hood etc on new adventures. Only read one so far but it went down well. Was about 150 pages I think? Pictures of most pages but language a bit of a step up from Kitty / Isadora Moon.
The Worst Witch might be too hard now, does have some hard words, but maybe with you supporting her? Magic school for witches, follows Mildred for whom everything always goes wrong but right in the end! Pictures on most pages, only about 100 / 150 pages long.