I have picked pumpkins off the vine at several pumpkin farms in the greater Seattle area. How to get it off the vine probably depends on the pumpkin variety, but the way I do it is to twist the pumpkin until the stem breaks off the vine, no sharp tools needed.
My kids weren't able to pick the pumpkins off the vine until they were around 5 to 6 years old, so when they were younger, they would tell us which pumpkin they wanted and my husband or I would pick it off the vine. Or tell us which one they wanted from the piles of pumpkins stacked around the farm. Either way, I have never thought that they didn't picked their own pumpkin.