Pre-Witched - S3-E17
Other mistake: During a flash back of Grams in the kitchen she puts a rose in a pot, then Piper walks in, they talk and Piper starts to add some salt and a couple other ingredients into the pot. Grams asks how she knows what to add, Piper says says something about mustard - Piper wouldn't know what's in it already, she just walked in. And she never even finds it strange that there's a rose with a stem in it.
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.