I am disillusioned enough to think I can make a bed for my son (he's
three, it's his first bed).

I am trying to make a loft bed. I'm using 4x4 cut to 3 1/2 feet for
the posts and 2x4 to hold the posts in place. I'm gonna notch the top
of the 4x4 so the 2x4 fits in/on the top. I'm covering the whole thing
in 7/16 OSB that will form the lip that holds the matress in place.

I feel I need more support for the posts than just one piece of 2x4 at
the top. I was thinking of notching the 4x4 near the middle and
placing another 2x4 there as well.

My questions a
1) Will the OSB provide enough stablization or do I need the extra set
of 2x4's?
2) Will notching the 4x4 in the middle weaken the 4x4 even though I'm
going to put a 2x4 in the notch? This is for a three year old and he's
going to jump on it I'm sure.
3) If I'm just totally lost please let me know what I should be doing
to reinforce these posts.

Thanks for then help.

Tom P.