D.W.
Prilosec is one of the most prescribed drugs in the world.
Zantac, as Koren mentions, is an H2 antagonist. Prilosec is a proton pump inhibitor (the current form is called Nexium as they were able to isolate the active part of the molecule to make it stronger). Zantac is an older medication, largely regarded as less effective and not proven to be any safer.
As your pharmacist regarding its use in children his age. I would have NO reservations about using it in my own children. Most side effects are minor, but you may be able to find more information specifically about the pediatric population vs. the majority of its use which is adult.
Good luck. If you have a choice, I'd always recommend a proton pump inhibitor (PPI) over an H2 antagonist like Zantac, Axid, etc. because they are simply more effective for more patients.