someone who can do anything they set their mind to
「やればできる子」は英語では「someone who can do anything they set their mind to」になります。
「頑張らないともったいない」ー> It's such waste if you don't try.
「やらないともったいない」ー> It's such a waste if you don't do it; It's such a waste if you don't give it a shot.
例文:
That girl is super smart. She can accomplish anything she sets her mind to.
あの子はとてもかしこいですよ。何も頑張れば達成できるよ。