Rant - setting boundaries

Tried to set some ground rules during this festive season by requesting my MIL (and her relatives) not to anyhow give food/snacks to the LO. Caught them still doing it and even trying to hide and feed discretely. Even to the point that I got called being over protective and too rigid. How else can I enforce my boundaries over my LO? I already got my husband to play the bad cop by calling them out when they get caught doing it, but it spoils the mood because they will just show a black face after that. I know they are not bad people and my husband really enjoys family bonding time. What else can I do? It’s so tiring having to keep a watchful eye while trying to enjoy the festivities. 🫠