Any stories where Spike and/or Buffy, become kids, be it physically or mentally, and are taken care of by the other one? I got the idea from a recently read Spuffy story where Buffy stays in her physical 20+ year old body, but her mind goes back to that of a four year old (due to a spell). So I'm looking for something like that or it can also be a physical change.

I think Strings by Kari Mouke fits, Spike becomes a child and Buffy cares for him. I don't know if it is online anymore but I have a copy if you want to pm me I'll send it to you.

Innocence Found by spikes_heart

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=20113

Summary: Still going through the motions of everyday life and slayage, Buffy and Spike come across a demon that changes their very existence. Complete

There is a sequel too.

Reclamation by spikes_heart

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=20347

Summary: Buffy wishes her friends could understand the changes she and Spike have been through. Complete

Just a Teenage Crush... by Addie Logan
Summary: After Buffy's day from hell thanks to the Geek Trio, Willow decides to "help" by making Buffy forget she ever died. And—big surprise here—this goes terribly, terribly wrong. (Less angsty alternative to season 6; goes off canon following "Life Serial.")
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=4776
Not exactly a child, but Buffy reverts to being a teenager in this one.

EddyxMarvoloxRiddle
I can't think of any off the top of my head, but could you tell me what that story you read was called? The one with Buffy as 20yo physically but with 4yo mind? Thanks