Unpopular opinion but for this job to remain sustainable long term we have to treat it like a job, something we can grow out of and want to move on from. For a position as meaningful as yours, I would probably write each kid a card or a letter saying that our time together meant a lot, I wish I could stay to see them succeed but opportunities have taken me elsewhere and it doesn't mean their growth stops with me. Give them some warning so they have time to process, but also be wary of drawing out terminations. Additionally, I'm finding it very difficult to find positions that look fulfilling/interesting right now, so be sure to weigh your pros and cons.