You could try selling this to them as something they can add to their marketing. After all it adds to the attraction of their venue if this service is on offer. If this isn't enough for them then try sweetening the deal with a small cut of the profits that way you only pay out if you make money. I have never done this myself but know others that have and its usually a case of making them aware of the mutual benefits that your being there will bring. Also make sure you have adequate public liability insurance in case anything goes wrong, something as simple as a child running into a light stand could ruin your whole day without it.
