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Getting the older to read to the younger does seem the natural thing to do, but my ds says he wants to be read to because it relaxes him and makes him feel sleepy. Ive noticed as my boys get older (I also have a teenager), they cling onto some childish habits. It's nice to you're still needed!
Little House in the Big Woods - fine for a four year old, but a 10 y/o will 'get' things that she won't.
The Katie Morag picture books? I still love the pictures and I'm rather older than 10
The old Wombles chapter books - again fine for a 4 y/o but very funny if you're older (you can get them on Abebooks)
Depending on your 4 year old, what about Professor Branestawm? (Is she just 4 or 4-nearly-5?)
Also, does your ds read to your dd? I know my 10 y/o loves reading aloud to small children (though she doesn't have small siblings, so it is more 'special' as its only when we're with friends). Come to think of it I have seen her and her 12 y/o friend happily giggling over friend's 2 y/o brother's picture books long after he has got bored & gone off to play
The Roald Dahl treasury has some great stuff to share - not all appropriate for a 4 year old but quite a few will appeal to both age groups. Also, I use Anthony Browne Picture Books (Walk in the Park, Changes etc) with my ten year old classes because they can get so much out of the pictures and that makes the book (which is appropriate for a four year old) fascinating for 10 year old. John Birmingham 'Oi get off my train' is good too because you can read it as a story or you can look at the bigger issue of animals becoming endangered.
Recently, my ten year old son has been asking to join his 4 year old sister and me when we are reading a bedtime story, rather than reading by himself in his room. I like us all 'snuggling' up together but I'm obviously finding it difficult to choose appropriate material. He doesn't mind listening to things that are a bit young for him and my daughter has a very good attention span for her age but it's still difficult. I've been depending on the 'Naughty Little Sister' series but I think we need something new. Any suggestions for a classic book of short stories?