This is a really wide open question. I'm going to assume that you are just going to cut the garments into squares, then add sashing, and make a 't shirt' type quilt, not actually do a pieced block. For a baby top, I would guess that you could only get a 6X6" square at the most. Toddler, maybe 10X10?? Lets say that you add enough fabric to the square cut from baby clothes to make up a 10X10" block, so you are working with all the same size blocks, then have 2" sashing
If a crib is about 45 X 60, you would need 4 garment blocks across, and 5 down for a total of 20 items. This, plus the sashing should get you in the ball park.
You will need to treat the knit items like t-shirts and add an iron on interfacing.
Don't know if this helps or not.