We had K's at a park too and did a smash cake for him and cupcakes for everyone else. We just waited until the end to let him have it and made sure to have plenty of wipes and paper towels on hand. We also had a change of clothes for him.
I would do it, there's nothing like watching them tear thru a cake for the first time! I would bring a change of clothes and plenty of wipes, paper towels, etc.
Re: smash cake at b-day party?
Kristen (7), Timothy (5), Robert (3), Charles (9 months)