Personally, I don't see that any of these things as compelling practical advantages, given that the kids already have dual Swedish and Belgian (and therefore EU) citizenship. If they were Moldovan and South Sudanese, that'd be a different story. Or if they were citizens of a country from which getting a visa to enter the US might be difficult in the future.
But most importantly I think this is one of those decisions that you just don't make primarily on a cost-benefit basis. It's not like deciding to join Costco or subscribe to Hulu. Citizenship entails responsibilities. If you want your kids to shoulder those responsibilities and feel allegiance to the US then it makes sense to get them that citizenship come hell or high water. But given that they already have two perfectly good citizenships from two advanced western democracies with generally positive international relations worldwide, I don't see much practical advantage in adding a third.
Still, I wouldn't presume to give advice, other than this. The poster needs to examine, very carefully, that feeling he has that maybe his kids should be Americans. The way he expresses it, "sentimental reasons", makes those feelings seem pretty trivial, in which case it hardly matters if they don't become Americans. After all, most other Belgians seem to get along perfectly well without being Americans too. But if this is at all something he suspects he might seriously regret not doing, or if it nags him in ways he can't quite put his finger on, he needs to get to the bottom of that in a way random people on the Internet can't help him with.