Nothing you can do about replacing the chainring. This is a cheap model, not designed to swap out rings. The ring is riveted onto the crankarm.

Your easiest option is probably to get a wider range of gears on the rear wheel, although it's possible you've already maxed out what the rear derailer can handle.

In that case, perhaps you can look into a replacement crank with smaller ring, although idk what the market for children's cranks with shorter arms is like.