Children will not become what their parents expect.
Children will not become the thing their parents desire.
「～ものだ」は本当に説明しにくいですが「thing」や「what」や「something」のは意味が近いと思います。
例えば「Children will not become what their parents expect.」の場合は「what」が「～ものだ」ような感じです。
「Children will not become the thing their parents desire. 」の場合は「the thing」は「～ものだ」です。
両親の希望が指定されていない場合は「something」も使えます。
Children don't usually turn out as their parents think.
Children tend not to do what their parents think they should.
usually と tend to は「 」ものだと「 」が当たり前だよという表現と相当する意味です。
その「当たり前」の意味をもっと強調したいなら、obviously か of course でも使えます。
Children obviously won't turn out as their parents think を言うなら、もう書いてある答えよりその「当たり前」の意味をもっと強く表現できます。
思い通り as thought/as expected
That's just the way it is. / That's just how it is.
That's the nature of XX. / Naturally
It's that way whether we like it or not.
「こどもは親の思い通りにはならないものだ」= Kids don't do what their parents expect. That's just the way it is.
Children don't fulfill their parents desires. It's just not in their nature.
「当たり前だよ」 は「naturally / obliviously」と言う言葉を使えます。子供と若者は「duh」をよく使います。
Kids don't listen to their parents, duh. = Naturally, kids aren't likely to listen to their parents. = Kids don't listen to their parents. Isn't it obvious?
オーストラリア 
アメリカ合衆国 
