We just did a blanket on top of his fitted sheet and a small toddler pillow (in retrospect I'd just get a small travel pillow rather than spend the $$ on a "Toddler pillow"). In the winter, we use 2 blankets. Stachio moved to his toddler bed right about 2 yo, he too started climbing out at nap time.
I've also found that just some warm fleece footed PJ's work for my boys. They often kick the blankets off, so when it is cold we do fleece footed PJ's with a onesie/undershirt.
As for the supersoaker part...I'm not sure what to tell you, but I will say that when Stachio night time PT'ed I was very grateful for the recommendation to layer mattress pad sheet, mattress pad sheet...so I never had to make the bed in the middle of the night. You're already doing the size bigger overnight and all the other tricks I know of....so I'm not so sure what else would help on that front. Hopefully someone else can chime in.
Also...the best idea that I got when transitioning to a toddler/big boy bed was to us a pool noodle under the fitted sheet/mattress pad to keep the kiddo from falling out of bed. Our bed did not come with a rail and the aftermarket rails wouldn't work on our bed because of the "lip" created by the front board (it wasn't flush with the bottom of the mattress).
Anyway, for $1 it totally stopped him from falling out of bed!
GL!
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DS - Adopted by loving parents 1995
DS1 7/2009 ('Stachio)
DS2 9/2011 (Peanut)