I was going to suggest some of those arcade games that you can find on PSN, and racing games like motorstorm that allow motion movement, my 4 year old cousin loves it. This before i knew she had the mentality of a 5 month. In that case i can't recomend you anything really, as far as videogame consoles go.
I'm not trying to tell you what to do, but i wouldn't get past showing her animated movies and T.V cartoons, and play directly with her. I don't see how, if she has the mentally of a 5 month old, a game system is ever going to work.