Why not get a sand pit for the younger ones. My grand daughter plays for hours with the sand. She is so taken with it she could be anywhere and would'nt notice. The one we have has a lid on it and i am sure you could put it in the shed when going home time came. If they knew they were going to the allotment to play in the sand it might encourage them
Another thing i do is bring a picnic. She now associates the allotment with picnic time and cant wait to go.
Don't have an allotment myself but have to travel to my place of gardening so its worth it.