I've always wondered about that ever since I was a child.
I've often wanted to ask about that since I was a little kid.
子供の頃から今までずっと疑問に思っていたわけですから、現在完了形(継続)を使って言えます。
ーI've always wondered about that ever since I was a child.
「子供の頃からずっとそれがいつも疑問だったんだ。」
I've always wondered about that で「それがいつも疑問だった」
ever since I was a child で「子供の頃から」
ーI've often wanted to ask about that since I was a little kid.
「子供の頃からよくそれが疑問だった。」
I've often wanted to ask about that 「よくそれについて質問したかった」=「疑問だった」とも言えますね。
since I was a little kid で「子供の頃から」
ご参考まで!