I wouldn't be making any 'deals' with a five year old. When we were taking our own DC to these places the gift shop wasn't an option and DH took them to the car. I did look around and if there was something suitable I would buy a small gift for the three DC. When they were little it was a badge (we pinned them on the bedroom curtains) or a postcard or two for their album. That was enough along with any photos. When they got a bit older and wanted to collect keyrings I would buy those. I could never see the point of buying plastic tat, rubbers, pencils. They did get to do that on school trips.
We occasionally take the DGC to the zoo or a playfarm and we do let them look around the shop. We do buy a small gift. No soft toys or anything the parents wouldn't want, and we don't spend a lot. We are the grandparents now so we can treat them.
A quote for those blessed with a spouse:
Recommendations for fans please
Did it have strands of gold running through it?