By the phrase double flush I persume you mean when you depress the flush hande it begins a normal flush, the water goes down in the bowl, then rises high again, and then goes down.
If that is the case, count the kids blocks. I would bet there is still one floating on the water in the trap. As the flush begins the water level in the trap rises, the block floats up blocking the flow which causes the water to again rise in the bowl, and slowly it will seep past the block until the bowl level is correct again. That is a common occurance when a floatable object such as a childs toy or a pill bottle gets stuck in the trap.