In situations such as you describe I find that I really have to think differently about quilt design. I know of nothing that says you have to keep even size borders. Only opinionated quilters will advise otherwise. I suggest this. Think about what your functional requirements are. I agree with the above post about the width. You really have a twin size in width. Is this sufficient for future use. What is the bed size being used now vs in the future. It might not be possible to make a " one size fits all ages". If you do make different width borders then select colors and width to blend in with the rest of the quilt. Make them look like they belong and not just added to make it longer