You left out the volume of the teaspoon....
But if the teaspoon is the standard
It really is a bit of a schemozzle when we mix metric measures, with a child's mass in pounds. I think it is just asking for trouble. Errors in dosage always occur, and this whack mixture of units would compound the problem.
The child would receive a dose of 0.15 g of ibuprofen.
Note: The answer can have only two significant figures, because that is all you gave for the umber of teaspoons.