Am I being unreasonable by having seperate food cupboard for my son?

My partner and I have been together for 2 years, living together for 1 year. He has 2 kids (7m & 4f). I have 1 son (7m). We both work full time. I have my kid full time as his dad doesn't see him. My partner has his kids every Monday, Wednesday for 4 hours after finishing work and then we have them Friday night for a sleepover until 20:30/21:00 on Saturday. It's my partners house which he had before me so he pays all house related bills such as mortgage, gas & electricity etc. I pay for my own expenses plus grocery shopping every week. As we have 5 mouths to feed 4 days a week, I do try and do the shop on a budget whilst still getting the kids treats and food they generally like.

My son is extremely fussy and I make him packed lunches every day for school with food he actually likes, so I keep his stuff seperate from everyone else so that he doesn't run out and doesn't go without. Bearing in mind that I buy a variety of good things for everyone else that my son won't eat.

I have noticed recently that things in the cupboard have been disappearing and I'm unsure if my partner has been eating them or if his kids have been eating them. Still trying to figure out the blended family dynamic so I'm trying to figure out if I'm being selfish and unreasonable or if my annoyance is valid?