If it has a sour smell, you've probably got:
- Too much nitrogen, not enough carbon -- add more cardboard & leaves. Avoid adding food scraps, lawn clippings, etc until the problem is fixed.
- Not enough air -- turn it.
- Too much water -- don't add any more water. (This also contributes to "not enough air" -- when it's too wet, the bacteria can't breathe.)
If you use just shredded cardboard (which has a very high carbon percentage), I'd add about 10% the volume of the bin to get it to equilibrium. If you use leaves instead of cardboard (high carbon, but not as much), I'd triple that.
This is a rough, off the cuff guesstimate. You'll have to add the cardboard, turn it, let it sit for a couple of days and observe. The cardboard will also soak up some of the excess moisture. If the smell goes away and the pile starts to heat up, then you've got the ratio about right. If it still stinks, add more carbon (leaves or cardboard), turn, and wait another couple of days. If the smell goes away but it doesn't heat up, you may have gone too far with the carbon. This is easier to fix than too much nitrogen: start adding kitchen scraps gradually.
If you have a readily available carbon feedstock (aka "browns": newspaper, cardboard, dry leaves, sawdust, etc), tell your kids that they need to add some whenever they add kitchen scraps. The "browns" belong on top of the scraps. Use more leaves, less sawdust -- the chart on this page will give you an idea of how much carbon is in various ingredients. (But your own experience will be the best teacher.) A thin sprinkling of garden soil over the top of the browns helps inoculate the compost with good bacteria and trap nitrogen from escaping into the environment.
(As a side note, "bugs" are expected in your compost -- they're part of what will break it down. But it's possible that you've got more than you should. They will probably go away when you get the mix right, especially if you bury the scraps under browns as described above.)