My 2 year old has a minor cold. can i give her 2.5ml of children's mucinex cold, and still give her prescribed 5ml of children's zyrtec? she is 33 lbs

Better not: Children's Mucinex cold has more than one formulation, and some contain an antihistamine as zyrtec, you don't want to combine both, besides some ingredients in these formulations may have side effects on your toddler. As for the zyrtec, 2.5 ml (2.5mg) would be enough, saline nasal washes, acetaminophen (e.g. Tylenol) would be helpful, together with rest and fluids, if any problem, call/ see her MD