What do i do if a three year old recieved 7.5ml of diametapp that was expired 2 months ago?

Dimatapp: It probably is ok. But i would now discard the medication, knowing that it is expired.

The shelf life is most likely extended, but in this age group cough syrup should be avoided if possible. There is one natural brand with honey for younger patients, but very often nasal saline is effective in reducing post nasal drip. Watch for signs of shortness of breath, fever and worsening cough.