I have friends who have taken their young ones I think around 4 maybe 3 1/2. I would take M easily and she would probably love it besides if they try to rub her feet or anything since hers are very ticklish and she just becomes a flopping fish if you try to touch them. If you are worried you could always try to find one of those more kid friendly places where they have kid sized chairs. They seem like they would be much more understanding of a wiggle worm or short attention spans, but also be able to keep them involved and entertain more while you get your own pedicure.
we do toes at home all the time, and have done jamberry on her fingers once spunbutterfly. she deft doesn't sit still very long to let her toes dry..i imagine she would do better in a salon, though, because it'd be out of her normal element.
I considered trying to paint G's toes last summer until I painted mine for a wedding and she demanded I put them back to normal. I'm kinda hopeless when it comes to hair/makeup/nails.