doubt with recently born kittens

I have a doubt. Recently I went down to our summer apartment in the
south of Spain and discovered that a cat had given birth to four
kittens on the roof terrace. Not being very knowledgeable about the
world of cats, I asked one of my neighbours about whether they were
ok. He informed me that they would be fine as long as the mother was
looking after them. The mother came to feed them every day and they
never cried for food. Now here is my doubt - the roof terrace is a
hard place to get down from. It is walled in and these walls are 4/5
feet high. We have returned to the north of spain leaving them alone
down there. I presume that the mother will continue to look after them
and they will grow to be bigger but I was wondering at what age or
size they will be able to get down off the roof. In other words will
they be able to jump the walls or will the mother be able to take them
in her mouth and lift them over.