Every number here is a working estimate to check with your building authority. Whoever prints this page takes responsibility for what is in it.
Shapes and levels
Low, wraparound, two level, upper. 4 lessons.
A guard on a low deck. Change it. Space the pickets on this low deck so no child gets caught. 
Wrapping around a corner. Change it. Price the guard for this wraparound deck. 
Two levels, one deck. Change it. Set the drop between the levels, and guard it if it needs one. 
A second storey deck. Change it. Pick the posts and the guard height for a second storey deck.