You can pay for a private guided hike, or you can just walk out there. If the lava is in the Kilauea crater when you are there, you won't have to hike - the circle drive goes all the way around the Caldera - it will be right there. On this map, the area marked #7 is where the current activity is.
If you decide to get up close and personal, be very careful with your grandson. There is nothing to keep people from being incredibly stupid out there. You can walk right up to the lava and fall in if you want - and people do. When we were out there a guide (not ours) told us the previous week he had seen someone walk right up to the lava, break through the thin top crust and burn both legs off at the knees!

When we were there, we saw teenagers walk up to the hot lava and poke it with a stick....I was scared to death that I was going to see a terrible accident!
This is about as stupid as you can get: (IMNSHO)