Is It Messy? If so, will this deter my guest from participating?
No! Just be sure to have wet wipes and perhaps a towel or disposable towels/towelettes available so guest can removed the ink from their fingers immediately. In addition, have a bin or basket available for the used wipes. Lastly be sure to have someone assigned to let guest know the process as well as to facilitate and coordinate the fingerprinting process.
In regard to the lack of participation due to ink, we have several testimonies of adults who got an absolute kick out of the Fingerprint Tree activity once they understood the significance. If you’re still on the fence about it, as an option, you might want to consider getting leaf-shaped stamp pads and just having guest use it and then sign their name.