My brother and I had a written agreement for him to use part of my land. Sadly, he passed away a few years ago. Now, his children are claiming they can continue using my land based on that old agreement, even without my permission. Is this legally valid in India?

In India, an agreement for the use of land is generally personal to the parties involved. Since your brother has passed away, his children would not automatically inherit his right to use your land based on that old agreement. They would need to establish a new agreement with you for continued use.