Am wondering whether they instigate the visits or you do? Ie have they called and asked if the children would like to visit or have you implied/conveyed an expectation that they should have them?
My iLs asked us if the kids could visit, and when they first asked were very nervous we’d say no so explained how they’d keep them safe, take them out etc. They had an expectation of having the children for the odd week every year because my DH and his Dsis always went to their grandparents and the iLs were excited to continue the practice. We only asked them to have the kids once (I had major surgery, DH was away with work), but let them know they could have them whenever they wanted and just to ask and we’d make it happen. My children went to stay 1-2 weeks from the ages of 4 and 7 and both parties loved it. I have one of each, so not sure whether 2 boys are more of a handful,
It sounds, though, as though your iLs may find having to lively boys rather tiring and don’t feel they can entertain them at home for extended visits without taking them out - and that bad weather/winter visits make being stuck in doors more likely and going outside in shite weather a bit of a trial. Tbf two adults and 2 kids going to the zoo, cinema, petting farms have become extortionate over the last few years, so can understand why they would baulk at the cost especially if forcing themselves to do it on wet and cold days.
At the end of the day, if they’d really rather only have them for short visits in the summer/spring, I think they are within their rights. And if you can offer some money towards day trips to facilitate their stay, that would probably go some way to soften it for them - but I really would allow them to initiate when/how long stays are by asking them to contact you to arrange visits when they feel up to it.