盗人を捕らえて見れば我が子なり, ぬすびとをとらえてみればわがこなり
Expression, Proverb
the doting parent's purse is plundered, the thief caught turns out to be one's own son
子泣き爺, 子啼爺, 子なき爺, 子泣きじじい, こなきじじい
Konaki Jijii (a ghost in Japanese folklore with the shape of a small old man and having a baby's cry)