Can anyone recommend things to do with a 6yr old and a 2 yr old in Kona, Big Island?

Keiki beach! Kukio beach is the real name, I think. It has a lovely calm lagoon for kids to swim. It's in a gated community, so there's limited parking so go early in the morning.

Or later in the evening.

There's very little shade so bring an umbrella. We liked beach 69 the best, lots of shade trees, calm water but good snorkeling for the adults.

We went on a body glove dolphin boat and that was a lot of fun. Tickets are cheaper via Costco. Bring Gravol, we had choppy seas and they ran out. But they have rafts with a window for the kids to float around and look at the fish.

Costco also had the best price for us on a rental car, although we needed a minivan so that may be why.

Diana Good point! We only ever went there at around 4pm until sunset, so shade was never a concern!

I wish I could remember the place, but one of the tourist jewellery shops has a thing where the kids open their own oysters to get a pearl. My kids LOVED that.

My kids were 5 and 7 when we went, and they absolutely loved the submarine. It's $$$ but they'll remember it forever.

We also really liked the beach at Kaloko-Honokohau. There are ALWAYS sea turtles there and it's lovely and sandy and there's a good area for the kids. They sometimes have cultural programming in that park too, so check that before you go.

I'd like to join in this thread as we (4 active, mature adults) are off to Kona for 8 days in Spring. Any tips on snorkelling companies etc -- we want to do the Manta Ray swim. Also tips for exploring Mauna Loa, Hilo, Volcanoes NP and the like without spending a fortune?

David Rent your own car and you can go everywhere. Discount Hawaii Car Rentals - buy gas at Costco - get the Hawaii Revealed book and go
Details of every area - things to see and do
Take at least 2 to 3 days to drive the island. Distances are short but it takes time eith all the stops. Im the south is the most southern testaurant in the US - in Naalehu - called Hana Hou
Go to everywhere - all so easy and fun.
Best snorkeling at 2 Step also good at Beach 69 -mantaray snorkel is a bit od a gong show - trips to Captain Cook are good - very good snorkeling if you are not up for the hike
Enjoy#

You need to rent a car on the Big Island. No getting around that expense. If you are staying in/around Kona, you can do both Mauna Loa and Volcanoes NP as (separate) day trips. Volcanoes is closer to Hilo, though, do if you are going to stay over there, plan to do the volcano from that side.

Got the car rental sorted already -- happy to read any tips people may have.

None of the places you mentioned cost a fortune if you have a car and use basic frugal principles like bringing your own water and food. I can't even remember if there are park fees, but if there are, it's not much.

The Place of Refuge is lovely.

The kids liked the sand floor at huggo's on the rocks, but we found most of the restaurants were very kid friendly.

We also went to Huggo's with the kids, and enjoyed it quite a lot. Go at sunset!

Following

Watching Sea turtle at black sand beach . Kids can easily spend a day at the beach , but this island doesn’t have white sand beach like Honolulu or Maui do . Go target or Costco first thing you arrive to buy water shoes for kids and rash guards to wear around the beach . you can go Hilton taking their resort train and get off at main lagoon , swim with dolphin and water slides . You don’t need to stay there to use the facilities

there is a octopus farm and seahorse farm just an easy uber way. both very interesting. we went to the octopus farm you get to feed an octopus. pretty cool. you cnnot do both the same day due to possibility of contamination of the tanks