Their breakfast is eggs,milk,apples and bananas.这句话有错吗？

应该改为are,因为eggs是复数

There are eggs, milk, apples and bananas for the breakfast.

is,是正确的.
Their breakfast 是单数,Their修饰breakfast 而已, 主语说的是breakfast 而非 they.

没错 主语是breakfast 是单数 当然要用is

对,用is,主语是their breakfast 是单数,不要看后面的,这个不是there be句型

They have eggs,milk,apples and bananas for breakfast.