We did a couple of times, but only after 18 months. The first time the GP recommended we do so, he told us to give half the lowest dose.
The second time was when she was 22 months, we diluted a half dose in her water cup and she drank it over about 30 mins.
Both times worked a treat and only needed one dose, and she was perfectly fine.

NB: Charlotte has no other illness, and no underlying health issues, hence why the doctor suggested we give her the expectrant.