Omega-3s are reasonably safe, and there is very strong evidence they are very important for brain development. Do not expect miracles overnight, but they may indeed contribute to your child's learning. In particular, you want a supplement with lots of DHA.

I'm not sure of the proper dosage for a small child. You may want to ask your pediatrician or natural health professional. Just make sure that your supplement brand of choice is distilled and mercury free.