Hold on a sec!
This is a bachelor household that has hosted 30 people that they were so unprepared to host that guests had to do some of the cooking AND after meals they have the tendency to disappear and let their girlfriends and mom clean up?
And now, you want a strategy to help them keep the gifts you are giving them?
How much do these young men need to be saved from themselves?
First of all, while it may be nice for them to "host" the breakfast, they apparently didn't know enough or didn't care enough to be prepared to host it properly.
Secondly, if you are hosting, you don't disappear after the meal and let everyone else clean up. And thirdly, you take care of your possessions so that they are not appropriated by others, including and perhaps, especially by your own mom when her doing so is a poke in the eye to your cousin who cared enough to give such a thoughtful gift.
I don't think you can trust them to value these gifts enough to protect them from being taken. It's not just mom wanting it, its them not having a realization of what hosting should entail enough to know they should value it.
Woah calm down there

This is my family you're talking about here.
For a very long time only one household has hosted the major events and holidays in the family. Last year, the guys bought their first home together and offered to take on the Xmas breakfast instead of holding it at my Aunt C's home. The way our family works is that Aunt C offers the use of her home and each family brings a dish to share. It's worked very well for the last 15 years or so. Last year, the guys offered to do everything. We offered to bring dishes as usual and they said no, they could handle it. It didn't occur to us to ask if they had the proper utensils and cookware until after the fact. Just because our families version of hosting doesn't match what others think it should be, doesn't mean it's wrong.
This year they have asked for help and I am going over on Monday to help prepare. I was going to take everything over on Saturday, but it can wait until Monday
Their mother does not help to prepare food or clean up. The random girlfriends and leaving them to clean up is annoying, but they are grown men and I'm not going to dictate to them what to do in their own house.
They most certainly do value gifts that they receive, both tangible and intangible. However, in this case, I caught their mother going through their gifts red handed. If she hadn't been to our house and gone through things, I would have given them their gifts and not thought about it. However, I know she wants the griddle for sure and there were a couple of other things that had been moved to the top of the pile from the bottom.