There aren't any kid-friendly treks or trails that would result in seeing any lava except the remote possibility of nighttime glow from a distance if you go to the Kalapana viewing area and stay near your car.

I would probably suggest staying in the Waikoloa area, as you are not too far from Kona, have easy access to Hilo and the Volcano area via Saddle Rd, and can go north as well.