Plot hole: Throughout the series, it's established that good witches don't freeze. In this episode, however, Little Piper freezes Future Prue and Piper when they come back to the manor to "kidnap" their younger selves. Simply because the Charmed Ones reason that only one "set" of sisters (past/future) could have their powers doesn't mean that Future Prue, Piper and Phoebe aren't good witches. In other time-travel episodes, the sisters remain immune to freezing and are still able to say spells make potions, meaning they are still good witches. For example, in Season 8 "Forever Charmed," Piper and Leo go back to 1975 (the same time-period as this episode) using Coop's ring, trying to find when Phoebe is alive again. However, when a startled Patty tries to freeze them, only Leo freezes. Piper does not, though she likely cannot use her freezing/exploding power in this time.
Continuity mistake: In this episode, the sisters go back in time to late March 1975. On the family tree it says that Piper was born on August 7, 1973. So she would be 1 and 1/2 in March 1975. But she looks at least 2 years too old. (00:09:30)
Factual error: Little Piper (from 1975) has blue eyes. She should have brown eyes, like present day Piper.
Other mistake: Grams says that if she died the girls would automatically get their powers; but they got their powers 6 months later and Phoebe had to read from the book of shadows first. Not really automatically.
Revealing mistake: In the scene where Penny uses her powers at Nicholas and throws him on the dining table, it is clear that the table is a fake one simply made of simple wooden boards.
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.