Two short lines of sand and one of growing up, and the poem's whole idea sits in the comparison between them. A child holds sand in his hands and lets it run out, the way anyone might at a beach or in a sandbox. The poem suggests that this simple, playful act is also a picture of time passing. Each grain is a tiny moment, and none of them can be held back.
The word "Slowly" matters. Growing up isn't sudden here. It happens grain by grain, so gradually that the child doesn't notice it happening, and the poem has already moved on to "adult" by the time we do. The speaker seems to watch from a distance, gently and a little wistfully, without naming a feeling outright. The mood is quiet and reflective, with a touch of sadness. The child is just playing, but the reader sees what is quietly being lost.