I think you are a doll, to think of your neighbor's kids! And I totally see where you are coming from. I think you should set some boundaries.
But keep this in mind...
These aren't 'awwww poor kids having to do without'. Free weights, or a shelf, or whatever stuff you might want to give away, is a generous gesture, bt is not vital to a happy upbringing or a good quality of life for a child or teenager. Genuinely poor kids don't have nutritious food, or adequate clothing, or shelter, or caring family members who take a vital and lively interest in the lives and activities of their children.
So if you decide to put some distance between you and your neighbors, please please please don't feel guilty that these kids are 'missing out' on some free goodies. It's just stuff. What is more important is that you care for yourself, and your family, and that you have good feelings and positive energy in your life!
Hand over the weights with a smile - at your convenience - and be glad in your heart that you got rid of some clutter out of your home. Enjoy your time with these folks, enjoy their company for what it is, and don't expect more from them than you know they are realistically going to give you.
If you accept the relationships for what they are, you won't be disappointed, or feel used.
And for the record, I don't think any of this is petty.