Unfortunately, no, there is no way to lock down apps on a stock iPad so that others can't get into them. You can buy some apps (mainly photograph ones) that enable you to put a password on - but it's only for that specific app, not for any app you wish.
Now, if you were to jail break, you could install tweaks to either lock apps (and folders and layouts and...) or to set the iPad so that the Home button won't allow anyone to leave an app (will lock your littler one in to whatever app you choose).
But, those options are only if you are jailbroken. Unfortunately, for a stock iPad - there is nothing. Sorry.
Marilyn