Translation:I am happy that my children knew how to wash the dishes.
Let's say this translation is...sloppy.
First a note on the verb sapere: it can mean that you statically possess a knowledge but also the acquiring of such knowledge, generally imparted by others.
It's common to hear "ho saputo che..." that basically means "I heard that/someone told me that"
I would say "sono felice che i miei figli sapessero lavare i piatti": for whatever reason I could not do it but fortunately my sons had the ability to do it.
"Sono felice che i miei figli abbiano saputo come lavare i piatti" seems to imply that they were not able to do it before and that someone (not me) told them how to do it for the occasion.
"Found out" seem to imply that they discovered it by themselves so is not very accurate either, the best thing I can think of is "I'm happy they got to know it" even if I don't know if it is correct in English.
Well, actually, I believe Duo is having a bit of fun with the two cultures. Italian mothers would be proud that their sons knew how to wash dishes even though some boys might have thought it sissified. Maybe he (Duo) is also sending a barb at modern parents an all cultures who do everything for their sons. Duo deserves a lingot for that.
I would have said that it is not clear, or even likely, that the plates are theirs in this context. I had visions of them washing everyone's dishes after dinner for example. The possessive adjective can only be omitted if the implication of ownership is clear, and I do not think that that is the case here.
As usual, Duolingo translates "sono felice" as "I am happy". In this context it sounds a bit ridiculous, and "glad" would be quite enough !