It is a strange one

However I can not see how a SKY digibox displaying Cbeebies taken from a satellite feed can enhance/generate a signal that would meet the requirments of DTT reception.
It sounds like the RF output of the SKY digibox is being passed through and the resulting picture has nothing to do with the Freeview box at all.
Retune the SKY digibox RF output and then try to duplicate the problem, perhaps the RF output is too close to the frequency Cbeebies is using, but if that was the case it should be affecting the other channels on that mux i would have thought.