Eating Kiddush Leftovers and Other Common Acts of Theft

My shul rents out a room in a building that also has a hall. It is not uncommon for a celebration to be held in the hall on Shabbos and much of the leftover goodies (cake, cookies, etc.) to be left sitting there. When we come to shul for minhah services, there are some children and adults who feel free to take from the leftovers. In my mind, that is theft and I have told my children unequivocally not to eat such food. Here is a general rule: If it doesn't belong to you then don't touch it.

I saw that R. Yisroel Belsky agrees with me on this matter.

[UPDATE: A comment has convinced me that the following is incorrect. Rather, if you are ready to buy something and the seller will make a profit from the sale, you can use the item (i.e. eat the food) prior to actually buying it. See Mahaneh Ephraim, Hilkhos Gezeilah 4; R. Akiva Eiger, Glosses to Hoshen Mishpat 359:2 on Shakh no. 4.]

Another issue - eating food that you are about to buy but have not yet bought. For example, if you pick up a danish in a bakery and are standing on line to pay. The danish is not yours! If you eat it, you have stolen from the bakery. I make sure to get explicit permission in a pizzeria to eat before paying (if there is no one currently at the cash register and I plan on paying when I am done eating) and, on more than one occasion, have had to deal with kvetching children in a supermarket who want to eat something we have picked up. The kvetching may be annoying but the educational opportunity is priceless.