My son is 8 and I'm embarking on the same journey.

Right now I'm just letting him play with some of my models, but building up his tolerance for board games with complex rules.

We've graduated from Mousetrap so there is still a ways to go....

My kids love monopoly and  a few others as does their mom, my fiancée.

I *LOVE* all the little self-contained boxed games that GW does, and they're perfect for introduction to the hobby. I'd recommend looking for the Space Marine Adventures games too, paticularily Rise of the Orks and Doomsday Countdown (although these are all 1vs1 games). Additionally, although a little older, Blackstone Fortress can still be bought from GW for a great price and is up to 4P co-op (officially), and you can ham-fist in a fifth player to act as the enemy commander. 

18 minutes ago, Grotsmasha said:

I *LOVE* all the little self-contained boxed games that GW does, and they're perfect for introduction to the hobby. I'd recommend looking for the Space Marine Adventures games too, paticularily Rise of the Orks and Doomsday Countdown (although these are all 1vs1 games). Additionally, although a little older, Blackstone Fortress can still be bought from GW for a great price and is up to 4P co-op (officially), and you can ham-fist in a fifth player to act as the enemy commander. 

Great idea Grot. I really want to introduce them to space hulk bu it is still too pricey 

Love this. All great of my kids have been into 40K. My eldest is now grown up and still likes it but doesn’t collect anymore. My daughter isn’t mad keen but she does have a Necron army with she painted pretty well. My middle son is nuts about it and managed to start off half a dozen other lads at school in the hobby too, as well as turning the board games club into a Warhammer club!

Enjoy this with the kids; it’s an amazing hobby for children and is also so motivating for yourself as well.