First - if the person cleared the storage area and "left them by mistake" - that is the answer - it was their mistake - not yours.
Second if the stuff was there for nearly a year and no one expressed interest, I would have probably contacted the person if they were close and asked them to clear their junk out or I would dispose of it. But if I had not heard from them for a while, I would not hesitate to dispose of the stuff left behind for a year.
Third, as to new owner - it would be up to them as to whether they would entertain an offer from original owner - depending on the situation and how attached 5 year old has become to dolls.
But I would not give original owner their name - as I think it could get into a pressure situation that would be unfair to new owner.
If the dolls are particularly valuable, either sentimentally or antique value, I would think original owner might make a cash offer to buy back, or offer to buy replacement dolls chosen by the 5 year old and exchange. After all it was Original owner"s mistake that caused the problem and adults should be responsible for consequences of their mistakes. But I would explain to new owner and mediate the offer to protect the child.
As to your part, they could just as easily be in a landfill if the helper had not asked for his little girl, so I think both you and Helper should be above any guilt in this unfortunate situation. But above all, the 5 year old should not be hurt by this - she made no "mistake". And further, neither you nor the helper should be expected to contribute financially to the solution.
Thats my 2 cents worth.