① Dad, you should take the medical treatment, then you'll know how painful it is.
② Dad, if you tried it, you would know how painful it is.
「お父さんも自分で受けてみて、どんなに痛いかわかるから」は
① Dad, you should take the medical treatment, then you'll know how painful it is.
※medical treatment 治療
※how ~ it is どれぐらい~か
(※年齢によっては言葉づかいが難しいかもしれません。)
②Dad, if you tried it, you would know how painful it is.
お父さん、もしやってみたら、どんなに痛いかわかるよ
※if you tried it, you would ~ もしあなたがやってみたら、~だろう(仮定法)
でどうでしょうか。