There is no lava flowing currently that will be visible to little nonhiking kids, except from a boat (not a good idea).
A helicopter ride might do the trick, but you'll have to pay a lot.
For current conditions, updated daily, check out
http://hvo.wr.usgs.gov/activity/kilaueastatus.php

p.s. read the "Hazards" part of the briefing

If you go to the VC at the NP you will find plenty of information on what is happening when you get there. Do drive down Chain of Craters road, they can see hardened lava that has engulfed trees and where the lava flowed over the road at the end. When we were there you could see a spectacular plume of steam rising from where it was entering the ocean, a distance away.

They will love going through the Thurston Lava Tube! Don't let the (beautiful) walk down and in put you off; the way out and back is quite easy.

Thank you so much for all this helpful info especially the link. So helpful!
It's interesting you mentioned the boat because they have a few pictures on Hawaiian websites where they show people on boats pretty close up. According to what I read on the link it doesn't sound like a good idea.

If you're also going to be on the other side of the island I'll bet the wee ones would love the seahorse farm. You can stick an arm in the tank and the amazing little creatures wrap around your finger. I loved it--and so did my wee one (who was 50 years old!)

I think it's called Ocean Rider--but I'm sure you could find it if you do an online search.

Have a great trip.