Hello I am a solicitor with over 15 years experience I will try to help you with this. The answer to this will depend on the nature of your agreement with this lady, is there a written agreement?
I think the position is this. As far as the dog owners are concerned they have a claim against the lady who they dealt with but if they bring a claim than she will have a claim against you. Of course there will be no claim against you if there was no negligence. If the attacking dog is known to do this sort of behaviour by it's owner and they did not pass this information on then they may well share some of the blame. I can't give a definitive answer to this question because although the facts are quite stark the detail is not. Ultimately, if you have failed to take reasonable steps to ensure the safety of the dogs then you will be responsible at least inpart. If the actions of the aggressive dog were not foreseeable then you may well not be responsible.