It’s fine. Worrying about growing is certainly a concern of many small children. In this case, it is Muriel’s imaginary friend (small, green, rather alligator-esque) who notices that Muriel is growing and he is not. He tries planting himself in the garden; he tries having Muriel roll him like her play dough; he tries forcing the issue; and he tries tall shoes and hat. Nothing really proves successful, but gee, overnight he does indeed grow (even if he doesn’t actually catch up to Muriel, since she grows too). It’s really intended for the youngest readers, probably best suited to PreK or Kindergarten.