Toddler proof....

Guided Access:

http://learninginhand.com/blog/limit-an-ios-device-to-running-a-single-app.html

Built into iOS 6.0 and higher.

Not that would allow here to access more than one game or other app at a time, no. If the iPhone is running iOS 6, you can lock the iPhone to a single app via Guided Access:

http://support.apple.com/kb/HT5509

but you would have to reset that to change the app to which you want to give her access.

On the issue in general, and not to give parenting advice, but I really doubt the wisdom of allowing a todder to play with a device that costs several hundred dollars and could not withstand being dropped or thrown. You might want to consider looking for a used iPod touch for her to play with. That won't be any more rugged, but would be cheaper to replace or repair than your iPhone 4S.

Regards.

As I've already provided, Guided Access is the only way to do what you want.

At risk of parenting advice, as with varjak paw, even the child-ruggedized cameras (Fisher-Price does make them) are targeted at children aged more than 13 months.

Laugh and learn ... sound like this forum at imes. 😝