Notice the effort a child made when nobody had promised a prize for it.

Effort that arrives without a reward attached deserves more attention than the kind produced by a chart or a promise, because it came from the child rather than from the arrangement. That is the sort of effort that survives once the prizes stop, and it quietly stops happening if nobody ever remarks on it.

Say what you saw. Something like: you kept going even though it was tiring, or you did that without being asked. Describe the effort rather than praising the child in general, and do it near the moment rather than at some later summing-up.