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You must make a stew using 3 red flowers and feed the children. They’ll start exercising and after completing their “exercising program’ they’ll improve at building. The more you make the stew and you feed them the more they gain their building skills.
But you must be careful. They are 3 kinds of exercising: swimming (in the sea), jogging (around the island) and body building (using the stones near the gong).
If the sea is clean and the children go swimming they won’t complete their “program” because the children can’t swim.
If the sewing hut is completed and the children go jogging the hut will stop them (this is a glitch of the game).
That’s why they can gain their building skills only by bodybuilding.
You must take care of them. After eating the stew they must go to the place near the gong. If they go to the sea or to the pond (from there they start the jogging) drop them to the stew and feed them again.
If the sea isn’t clean the children don’t go swimming. So don’t clean it very early in the game.
Until completing the sewing hut the children can go jogging without any problem. So complete it as late as possible.
Good luck!