Make sure the "sugar free" isn't aspartame, sweet and low, splenda, or any other fake toxic sugar.
Xylitol, agave nectar, stevia or some other safe natural alternative is most important especially for children!
But, as far as "treats"... have you tried stickers???
Ok what we did with my daughter was we went to the 99 cent store and bought a bunch of little trinkets and wrapped them individually then put them in a "chest" in plain sight out of reach... then we would give her a sticker every time she'd successfully use the restroom, at the end of the day if there were no accidents or so many stickers... (you make the rules up to fit your family) we would let her chose her 1 prize from the chest and she'd get so excited! she would talk about her prizes and she'd do her best at having no accidents. The stickers we gave several each day but we'd have her put them on a piece of cardboard on the wall or something similar. we started letting her know that after her prizes were to run out she'd be allowed to do "big girl" or "grand prize activities" things like play www.pbskids.org, help in the kitchen, go to the museum, park, special dinner, go to movies or something like that. Then they feel like big kids because they can go potty all by themselves and chose to do grown up things too!!