My older child has been a champion picker since the age of 4. My younger child, now almost six, picks too. But then he eat what he picks.
My solution has been to give each kid a quart container, and then hint that anyone who wants a ride back home had better fill it. To the top.
Don't worry. My kids know that I won't really leave them at the paradise that is our local organic farm. But the empty container does help to give them a benchmark. Last spring I interviewed the manager of our local U-pick operation for a newspaper article, and her mother was even more hard core than I am. She told me the family rule was that you could only eat three berries while they picked. She's the tiger mother. I'm merely ambitious, for all the good it does me. The farm should weigh my younger son before and after we pick there.
If speed (rather than an idyllic day lounging among the fruit-loaded shrubberies) is your goal, try the milk jug method. Blueberries are easier to pick if you have two hands free. Cut a large portion of the top off of a plastic milk jug, leaving the handle entirely intact. Then, find some cord or a ribbon and tie it in a big loop around the handle. Make this long enough to go around your neck, such that the charton hangs with its open top at belly height. Now you can use two hands to tease the berries off the branches, dropping them into the milk jug as you go.
I'm all about the efficiency. Pies require quite a few berries. And pie is important. Thanks to the adorable kitchen nerds over at Cook's Illustrated, I now know to shave the flesh of one Granny Smith apple into the pie filling. The natural pectin helps the pie gel up.
And... "Hey kid! Put it in the basket, wouldya? I saw that."