At first you need to give the prizes immediately, to “hook” the students. You can then start to delay the prize to the end of the lesson or a convenient stopping point. Remember, the very short prizes of ten seconds can do wonders for reviving the children’s attention, as voluntary control over attention takes a lot of brain energy. See what works for your kids. If it works to delay the prize, that's fine. Delay becomes easier to manage as the students get older. It can work well to do the prize during transitions.