"La cuna de mi hijo es pequeña."
Translation:My son's crib is small.
Literally, yes, you would write "The crib of my son is small". In English we use the apostrophe to indicate possession (my son's crib) but in Spanish they use "de". What you've written is incorrect for several reasons. You've literally written, "My sons the crib is small". Hijos is the plural of hijo, so "sons", so you'd also need to make "mi" plural if you were to write "hijos" (mis hijos). But that doesn't make sense here. My son's crib is small = La cuna de mi hijo es pequeña. Although I'm not sure exactly what you're asking. What are you having problems with? :-)