I think Jml do a patch repair, which is specially made for pet urine patches on lawns. I'm guessing it's 100% animal friendly but not sure. Another thing to try is to dowse the area that they have urinated on with about 1\2 to 1 litre (1-2 pints) of water, to dilute the urine.
The reasoning behind this is:-
Dog urine contains high levels of nitrogen, which is what causes the grass to go brown. You should also notice around the edges of these brown patches that the grass is actually quite green and probably thicker than the rest of your lawn. The reason being that if you think about it the urine will be more concentrated in the centre and less concentrated out at the edges as the pool/puddle spreads out. This weaker amount will have a lower concentration of nitrogen.
Right so if your still with me after that garbled amount of typing. Nitrogen is one of the main ingredients, albeit in low concentrations, in lawn fertiliser. Therefore adding a large amount of water over the area should dilute the urine and along with it the concentration of nitrogen, which should be low enough to act as a fertiliser.
Simon and Poppy