Oh dear, a lot of us have been here!! Me included!!
You feel bad, because you 'know' it was not right. If he had taken your treats and eaten them, would you tell him off?
If he does know about it, you have to apologise.
However, there is a way to get out of this, if he doesn't know about it. Buy some more.
When you go to the shop, buy some treats for yourself too! Now, when YOU eat all your dinner, you can have a treat with your child, then you can be excited together. He will see this and it will be a positive thing for both of you :)
If you are dieting, treats are still important, but have you considered getting your little one to exercise with you? A walk before dinner/lunch time builds appetite, even if only for 10 minutes round the block. Then you can have your treat and feel good about yourself.
Life is a continual learning process and having children helps us continue to improve our own characters, and, eat more treats :)