Let's take an example. If you give a kid a book (which is just a single book, and not part of any series), even if the kid loved the book, there is no way you can guarantee that they'll be excited to read the next one. That's why series are a saviour. Once someone is hooked to the first book, they will automatically want to read the next one, and the next one, and so on. So, habit of reading comes better when there is a series!
I already wrote a post on which authors I used to loved as a kid: 5 Authors for Children. And, if you take all these authors- the stories are usually inter-connected. Same characters repeat, and this can help sustain interest.
All through my class III and IV, I used to love Blyton to bits. I think The Enchanting Faraway Tree, Famous Five, or Secret Seven is definitely a great place to start reading. But, for slightly older kids, I think classic series like Chronicles of Narnia, Harry Potter, Young Samurai, and the likes are the best. Fantasy is the best at that age. Like Anne (of Green Gables) would say it: lots of scope for the imagination!
I think I became a reader who can sit and read for hours and hours together when I started reading Harry Potter. So, once you can do that to one book, you'll get into that zone of a reader with a good reading habit! Another thing that my dad used to tell us and we always stuck to was completing at least half the book before we decide we don't like it. I swear by this rule because the two most favourite series ever for me- Harry Potter, and the Hobbit(leading to the Lord of the Rings)- I thought I hated them until I crossed a certain point in the book when I irrevocably fell in love with them!
Always encourage your little cousins, kids to read. It's a fun world, and it has made most of what I am today. If you want any suggestions specific to genre/other things, feel free to ask away in the comments below! I'll be thrilled to help!