What a difficult one. YANBU, not in the slightest. But they're not children. They've been alive for 70 years so they should be able to cope with a bit of a telling off by now. I think you need to be firmer with them, lovely though they are, as they're clearly not taking your current approach seriously.
Tell them that you feel they are showing little respect for you as a mother. Remind them how much DS loves seeing them, but that at the end of the day you are his mother. You have repeatedly asked them politely to show respect for your parental choices and they have repeatedly ignored this. Tell them that you find it very hurtful that they would treat you this way and that you understand that it's not their intention to hurt you. Tell them that now you want it to stop.
If this feels too heavy, how about the following:
Tell them a lie to back you up - the dentist has expressed strong concern about the state of his teeth, that they've become much weaker in the past (6 months? however long this has been going on). If they do not stop giving him sweets he will end up as the only child in his class (I'm assuming he goes to nursery?!) with fillings, or worse still, having his teeth pulled out (happened to 2.5-year-old daughter of a friend of mine recently), which is unacceptable in a 3 year old, yadda yadda.
Warn them that being so young he does not have the self-control to say no, so he needs them to be responsible adults and to put his health first. If they feel that they cannot do this, warn them that you'll have to stop him seeing them - it's their choice.
Easier said than done, but at the end of the day it's clear that if you don't change your tack they won't change either. Maybe give yourself a bit of Dutch courage with a glass of wine beforehand? Also I defo recommend rehearsing the conversation out loud - either with your DH or on your own. I recently did this for a tricky conversation (and I am always polite and kind to a fault!) and I was SO proud of the way I conducted myself. V firm, clear, and fair.
So you can do it too! Go on, girl!