try to make a game of it! and use a star chart. tell them,when you get to the shop, that your going to play a game the game is not to run away if they manage it they get a star tell them if they can get 3 stars they can have a small treat, but if they run away they get no star and stick to it, if they go to run away in a shop try and make it fun for them to stay with you try to get them to get things for you like who can find me a very red apple....ect