actually he had 8 children...
six from anastasia romanova (anna - 1548, maria - 1551, dmitri - 1552, ivan - 1554, evdokia - 1556, feodor - 1557), one son from maria temriukovna, his second wife (vasili - 1563) and another one from maria nagaia (dmitri - 1582)....
so eight kids out of which five were boys.....
his oldest son, dmitri, drowned before he turned one (apparently he was dropped in the river by his nanny
)
feodor was retarded, vasili died young and dmitri was the result of the seventh marriage which was not considered by the church (the orthodox church only allows three marriages).... and the tsar apparently caused ivan's death (the only one left who could qualify as an heir) when he kicked him with an iron (ivan jumped in the defence of his wife who was pregnant but was not wearing according clothes - or so the tsar thought)....
basically after he died there were two boys left: feodor and dmitri... feodor was tsar for 15 years but he was basically no good because he was retarded. dmitri died in 1591 (official version was that he had epilepsy, but there were many rumours of murder and even more rumours of him not being dead - there were three false dmitris and two of them actually ruled russia... now dmitri is a saint...)...
feodor only had a daughter, that led to a succesion crisis, civil war and there came the romanovs
...
does anyone get the feeling ivan the terrible sounds a lot like henry viii? just like him, he was obsessed with the succession, had many wives, fights with the church and in the end all that for nothing... there was still no proper heir... :-/