It is part of fostering that children will eventually move on. In many cases, this will be a happy event as the child will be going back to their own family, or moving to a permanent foster home. However, if you have become friends with them, or just got used to having them around, you will naturally be sad to see them go.
It may be possible to stay in touch with children who have stayed with you when they leave. Speak to your parents and the social workers to see if this can be arranged. These days it is easy to stay in touch on the phone and via email. It may also be possible for you to meet up.
Some families keep a scrapbook with photos and information about all the children who’ve been to stay with them. This is a nice way to remember people, and it can also be good to show new people who come to stay, to tell them a bit more about your family.