For my in-laws 50th (50!) anniversary the whole family went to Kauai and stayed just past Hanalei on the North Shore. If going to Hawaii, I 100% suggest to take the grandparents! Well, if the kids are a bit older than mine (1 and 3) then they could participate in all the fun Hawaii has to offer. Otherwise, then can play in the sand and waves with Grammy, while you hit the beautiful but muddy trails as well as the rafting and kayaking.
Suggestions for an awesome stay:
Tunnels beach: on the north shore, this is a great place for children. close to the beach it is calm and shallow. Waves break further out for your surfing needs. Also some good snorkeling and tide pools close by. A one stop shop when it comes to beach adventures!
Napali Coast: We left the kids with grammy and hiked to the first beach on the Napali Coast. I would love to do the whole thing one day, but little 1 year old Indi wears out the grandparents pretty quick- 15 months and still not walking, he is a back breaker! I was also concerned about mud and ups and downs on this trail. In hind sight it would have been do-able to bring them along, though easier without them. Muddy in some parts, but not as bad as a feared. J of course hiked in his Rainbow sandals (as he often does) and eventually just went barefoot.
Waimea Canyon: Quite a drive from the North Shore but the views are beautiful and if you keep going there are many trails to choose from.