Ask your children to make any notes and marks in their books in pencil or on sticky notes, not in ink or with highlighters.
Erase all the pencil notes.
Donate the books to local schools, libraries or thrift shops. Alternatively, ask friends with younger children if they need the books or offer the books on Freecycle.
Arrange a book swap in your community. Advertise the swap on A4 posters made on a computer. Make simple posters with very large font size in a word processing program and include date, time and place. Put the posters up in local stores, libraries, religious centers and schools. Tell your friends and neighbors and any groups you are involved with. The more people who participate, the more likely everybody is to get books they want or need. Note down some simple rules to prevent arguments -- for example, you can swap a hardback for a hardback, two paperbacks for a hardback and so on. Donate any leftover books after the swap.
Sell the books. Parents always need schoolbooks, even if they aren't the latest editions, and there is a thriving market in used textbooks. Investigate local secondhand booksellers or advertise on a site such as Amazon or eBay. Be honest about the condition in your description of the books. If they contain torn pages or a lot of notes, say so.