Dress in white...shirt and pants with one of those white "boat hats" ice cream men used to wear years ago. I don't know what they call them...they remind me of what stewardesses used to wear, only these are white like for working in bakeries.
Gives the appearance of crisp and clean.....and if you're standing in the road at night...drivers will be able to see you...;D.
If you were thinking of something to decorate the truck with...have a double popsicle dancing as a character...the 2 sticks could be the legs and you could have skinny arms and a face on the popsicle with a big smile.
I agree... strange question. The truck and the music/bells, etc. is what should attract children. In fact, you shouldn't JUST want to attract children... everyone loves ice cream. A costume is overkill, and could work against you. You want respect, not laughter. Just wear nice, casual clothes. If you want, add a hat appropriate for the job, but no costume!