[rwest]

When you take the tires and tubes out, Mark or note there relation to each other, and the wheel, then fill the tubes and immerse in water to see where the leak is, that way you can check the area on the tire for glass, metal or spoke/rim issues, otherwise you need to inspect every inch of everything- not a bad idea to do a quick check anyways, but being able to zero in on the spot will be very helpful.

When you put the tire/tube back in, fill the tube slightly- just so it has shape, maybe 5lbs, put one bead of the tire on the rim, stick the valve stem into the hole(make sure stem stays straight,) push the tube into the tire, then starting at the valve, start seating the other bead- DON'T use any tools to seat the tire as you will almost always pinch and puncture the tube, just use your hands, feet and curse words if needed.

After it is all back together, squeeze the beads on both sides as you work your way around- one side at a time making sure the tube isn't pinched, then fill up to full pressure- probably 50 to 70 on a mountain bike, check the tire for rating.

Last thing, good on you for being active with your kid!