I believe you are looking for Little Child (http://www.fanfiction.net/s/4252558/1/Little_Child) by authoressnebula but you seem to be mixing up two stories. The second one you describe is actually A Hiccup In Time (http://www.fanfiction.net/s/4809285/1/A_Hiccup_in_Time) which has the reverting back and forth and the bathroom scene.
And I also have a De-aged!Sam (https://delicious.com/cherry619/de-aged) tag on my delicious.
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Date: 2012-03-14 11:42 am (UTC)And I also have a De-aged!Sam (https://delicious.com/cherry619/de-aged) tag on my delicious.