I wouldn't be to worried if it's just a 1 off now and again. I would say next time can you offer some side veg or corn on the cob with dinner x
My mum is completely the same and she has my 18 month old And 3 year old 2 days of the week. I joke with her about it but I just try to make it up on the other days. They barely eat any dinner that I cook on the days my mum looks after them. Yesterday she took my 3 year old to the cinema and let her have 3 nomo chocolate lolly’s before the adverts even finished!
I know you dont wanna seem naggy but don't think there is anything wrong with being "that mum" when you're just trying to look after your child's health and establish some good eating habits. Sweets and treats basically make normal, healthy food taste like shit in comparison. 🙈 And so by them getting the enjoyment of giving him treats, it's making your life way harder. I would politely say that you're trying to cut down on treats so ask them to limit to only 1 per visit, and to serve veg with every meal. It's really not much to ask.
My mum is the same my eldest will come home with pockets full of sweets it hasn't gotten any better and she's 9 now 😳