Corrected entry: The past versions of Patty and Penny are amazed to learn that their daughters and granddaughters, respectively, are the Charmed Ones. However, both women knew the centuries-old prophecy that three sisters would eventually be born into the Warren line, and that they would be the Charmed Ones. It's conceivable that Patty, who came from a time before the birth of Phoebe, would not have thought of the prophecy at this time. Penny, however, came from after Phoebe was born, and previous flashback episodes had shown that Penny knew that the three sisters were the Charmed Ones immediately when the third sister was born, and was still convinced of the fact up until Penny's death [despite the birth of a fourth daughter to Patty].

Charmed (1998)
1 corrected entry in season 8
Plot hole: When Gith takes the potion from Chris, he ends up stomping on it and shattering the potion bottle - making contact with the potion. We've seen that potions don't have to be thrown by someone who's even a witch for them to work (like when Cole's powers were stripped etc) so why on earth would he do this? And how did it not vanquish him? There are other ways he could have gotten rid of the potion that didn't involve a way that really should have vanquished him.
Cole: Phoebe, I love you. I don't know what's going on but maybe I can help. Would you like me to kill someone for you?
Trivia: Holly Marie Combs's real life pregnancy with her son Finley Arthur Donoho was mirrored by Piper's pregnancy with Chris in this season.
All Hell Breaks Loose - S3-E22
Question: Later in Charmed they bring the so called "cleaners" in episode 6-2 when Wyatt conjures a dragon and magic is exposed. Why didn't the cleaners appear earlier in this episode when Piper and Prue exposed magic?
Answer: That was a mistake in the series.
Also, the episode is called 'Babies First Demon' and this alone tells us that it's Wyatt's first time in the field (so to speak) and as the sisters say, they've done a good job of clearing up their own magical messes so far. Nobody really knows how advanced Wyatt's powers are, or will become. That's why the cleaners appear, to nip things in the bud. The Cleaners didn't want to chance that the sisters could clean Wyatt's magic up, just incase they couldn't. Therefore intervening themselves, just in case.
Answer: Because they wanted them to fix their own mistakes.
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Correction: Yes, perhaps Penny did believe the girls would eventually become The Charmed Ones, but this wasn't confirmed to her [officially]. For all she knew, one of them would refuse their powers, die young [before their powers were restored], etc. Also, she died immediately after making a potion to strip their powers [Re-witched, Season 3]. She could have simply been expressing amazement that her beliefs were confirmed about her granddaughters' destiny. After all, she does say "I always knew The Charmed Ones would come from me...well, once removed, of course. It's one thing to believe a prophecy will come true, it's another to actually witness Future Piper confirming her belief.
jademoon
The original post is correct. Penny did know they'd be the Charmed Ones as soon as it was known that Patty was having a third child. That's the entire reason the warlock Nicholas made Patty bless the ring, and why Grams' bound their powers. It's been stated in other episodes as well. So Penny definitely knew, but in Forever Charmed she was shocked when Patty told her the girls would become the Charmed Ones.