infant motrin is a higher concentration than childrens motrin so it's easier to give them a smaller amount. you should be able to use either as long as you follow the dosing instructions for THAT medication. they will not be the same amount! call your pedi and ask the nurse for the correct dose for both medications.
I switched to childrens motrin around that age, ryan was not letting me shoot the liquid from the dropper into his mouth anymore, he wanted to see it for himself. I use childrens motrin with a measuring spoon (looks like a test tube with a lip on it) you can get them FREE at any pharmacy counter. he likes to hold it with me and drink from it.
you know how kids like to feel empowered, like they're doing their own thing and it's THEIR idea. :o)
~ t