My grandson who is three and a half eats a pack most days but his other snacks include bananas, apples, raisins, blueberries biscuits and malt bread. He tends to eat lots of snacks and quite small meals however he seems healthy and mostly has plenty energy. He isn't overweight so I'm not worried at all. I'm sure as he gets older he'll eat a greater variety at mealtimes but for now he eats plenty fruit even if his vegetable consumption is a bit limited.
He was breastfed until about 20 months so certainly had a healthy start. He was quite adventurous when still a baby but seems less so now. However thinking back to how picky his dad was when younger, he grew up eating pretty much everything and he now cooks most of the family meals from scratch when not away working. My baby GD is quite adventurous with food and is still being breatfed too.
I'm sure my DiL has had suitable advice and I try not to interfere in any way. The one thing that irritated me most with my MiL was her interfering with what my daughter ate and drank. My daughter was breastfed until about 13 months but never liked drinking milk after weaning. My MiL was obsessed with giving her milk in 'her daddy's special silver cup' from which she would drink a small amount to my great annoyance as she wouldn't touch it at home! Grandma would shriek with delight "See she loves it!" My health visitor assured me it was nothing to worry about as she ate loads of yogurt and cheese and things with sauces made with milk.
Years later my ex husband discovered he was lactose intolerant and he cut out milk altogether. When we told MiL she was utterly horrified! MiL insisted I had taught DD not to like milk because I didn't despite me trying really hard to get her to drink it. DD still eats loads of cheese and her coffee of choice is either a latte or a flat white which I can't stand!
If your GC seem healthy, energetic and not overweight, don't interfere. If you offer fruit snacks at your house that's fine, but never interfere when they are at home. They may try things at your house, as my children did, but at home they may demand what they are familiar with and refuse the healthy things they accept from Grandma! If you find they like soft fruits you could always ttake a punnet with you when you visit and say you just brought them with you as a treat.