If the floor around the toilet is getting wet AFTER you flush but water is not draining down the outside, it's not the tank to bowl, it's the wax seal. This needs fixed ASAP before you destroy whatever is below the toilet. Check flange to make sure it's not cracked and change the bowl to floor bolts. Don't ever reuse them. Based on what you've described above though, it sounds like water is leaking from the tank DURING the flush. There's a tank-to-bowl gasket that is probably not sealed all the way. There's also gaskets on the bolts that hold the tank to the bowl. One of them may have been compromised. It could be the flushing mechanism too that's spraying water during the flush.
Again, it sounds like the wax seal is ok, it's probably one of the tank gaskets. Just replace them all. If the boy is leaning back on the toilet while he flushes and the tank to bowl bolts aren't tight enough, the tank could actually be moving back which would cause the gasket to leak. Just check everything and replace all gaskets. If significant water is under the entire bowl, you're going to want to remove the toilet, let dry for a couple days, and replace everything. Once the subfloor gets wet and moldy, you're in for an expensive surprise!