My child forgets words they've learned after a while.

Words take hold by being used continually in everyday life. Even the same word (e.g., "train") — seeing the real thing, seeing it in a picture book, watching it in a video, hearing it on an outing — becomes better understood and harder to forget by being encountered repeatedly across various situations and contexts. And even when it seems forgotten, in most cases they'll come to use it again, so there's no need to worry too much. Think of it less as "re-teaching" and more as "scattering it throughout daily life." Hearing problems (such as otitis media with effusion) can be behind it, so if trouble retaining words continues, checking hearing is also recommended.