Babysitters Club, Bailey Schools Kids, Wayside School (Louis Sachar, who did Holes but Holes is a little older) and especially Magic Tree House are all great series for early readers
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Robert Munsch (I really like Paper Bag Princess) is also good for solo reading for early readers)
If they're strong readers and into fantasy, Deltora Quest series is a really great way for kids to get into epic fantasy, but it skews a little old for most 6 year old solo readers. Huge hit for the 8-10 crowd though
Same with Septimus Heap.
Roald Dahl was a bad dude, but Matilda and the BFG are still great books.
One Piece
The little prince has been done to death, but there's a reason it's so popular.
Warrior cats and wings of fire are cool. Dogman is popular and pretty funny.
Calvin and Hobbes is timelessly funny, but does have some adult themes.
Imo if the dad leads chud, you're better off supporting a love of reading in general.
I love Wrinkle in time, but it's pretty anti-communist
A wizard of Earthsea. Might be a tad old for them but not by much and Usula K Le Guin is super based.
Also A Wrinkle in Time was a favorite of mine when I was a kid
I literally came to recommend A Wizard of Earthsea. I think it would maybe be a little hard for someone of that age to read on their own, but if an adult is reading it to them I think they'd love it.
Just don't let Dad read the author's notes, she's pretty explicitly based in those.
Props to earthsea, the first one in particular is very accessible.
I haven't revisited them since I was a kid so no idea if there's any... themes but I read a lot of the Redwall series by Brian Jacques as a little one.
I'm just realizing I read more then than I do now 
At that age, especially considering potentially being read to: The Phantom Tollbooth
The Strega Borgia chronicles (starting at 6 or 7) and then into Series of Unfortunate Events (which is a really stellar way to build vocabulary in young readers).
Dr. Seuss books are very accessible at that age. I particularly like Oh The Places You'll Go. Richard Scarry's Busytown is another fun series. Then there are the oldschool Magic School Bus books. Another all time favorite is Dinotopia just for the imaginative worldbuilding of a utopian society. As they grow older you can't go wrong with Roald Dahl.
I'm also going to second A Wrinkle in Time.