As long as it's oatmeal with nothing added, I don't see why it would be a problem. Is it just oatmeal that has been ground up? I buy rolled oats in bulk and then run them through a coffee grinder until they are basically powder before I use them in soaps. Oh wait - I think I know what you're talking about - the baby's first solids type of oatmeal that you buy in a box and add breast milk or formula? Hmmm, I don't know about that. It seems like it would just dissolve or turn to pudding - that's what it always did when I added liquid and fed it to my kids!
It is oatmeal ground to a powder. If you buy it in the store, it can be $$. I see many who say they grind it in their food processor or coffee grinders. It is available at several of the soap supply vendors.
Could the baby oatmeal be used to make oatmeal milk to be used in soap? Does baby oatmeal have everything big people oatmeal has in it?