The fact that Seraph and Sati failed to hide from Smith does not imply that the Oracle was unaware that Smith would assimilate them. The Oracle told Neo that Smith would try to destroy everything (which would include Sati):
I see the end coming. I see the darkness spreading. I see death. And
you are all that stands in his way...Very soon he's going to have the
power to destroy this world, but I believe he won't stop there; he
can't. He won't stop until there's nothing left at all.
The Oracle probably had Sati leave because she had bonded with Sati and it would be traumatizing for either her or Sati to see the other assimilated by Smith (whoever was assimilated first).
Likewise, the Oracle's question about Sati does not imply that she was unaware of what Smith had done to her. The question is phrased such that the Oracle knew Smith had done something to Sati, and it doesn't take an Oracle to figure out that Smith assimilated her. Smith's choice to assimilate Sati is obvious and easy to understand, so the Oracle would have had no trouble figuring it out.
So why ask the question then? The reason may simply have been to provide an opening to express her disapproval of Smith:
The Oracle: What did you do with Sati?
Sati-Smith: Cookies need love like everything does.
The Oracle: You are a bastard, you know that?
Smith: You would know, Mom.
The reason may also have been to feign ignorance in order to hide the capabilities and limitations of the “eyes of the Oracle” from Smith. Just before the Oracle asked about Sati Smith indicated that he didn't quite know the capabilities of the “eyes of the Oracle”:
We meet at last. I suppose you've been expecting me, right? The
all-knowing Oracle is never surprised. How can she be, she knows
everything. But if that's true, then why is she here? If she knew I
was coming, why didn't she leave? [sweeps plate of cookies off table]
Maybe you knew I was going to do that, maybe you didn't. If you did,
that means you baked those cookies and set that plate right there
deliberately, purposefully. Which means you're sitting there also
The Oracle knew Smith was about to see his future victory over Neo (that's why he laughed maniacally after assimilating her) but she needed to hide from him the fact that although she could see Smith's victory she could not see beyond Neo's irrational choice to give himself up. If Smith knew he would be victorious yet be destroyed when he assimilated Neo then he would know not to assimilate Neo. The Oracle's question about Sati helps keep Smith confused about the nature of the “eyes of the Oracle”.
It is also worth noting that Seraph leaving with Sati is symbolic: Seraph protects the Oracle (“that which matters most”) but now abandons the Oracle to protect Sati instead – indicating that Sati will be the Oracle's successor. This symbolism would be lost if Seraph and Sati just sat around eating cookies (no cigarettes for little girls!).
On Seraph's part, he was clearly surprised that Smith had arrived so quickly but there was no harm trying to hide. Seraph likely reasoned that since Smith was there for the Oracle he could be distracted enough that he wouldn't look for Seraph and Sati. Unfortunately, there was really no place to hide. As explained here the only truly safe places to hide from Smith are the Architect's room and Mobil Avenue, but Seraph cannot access the Architect's room and the Merovingian would never allow Seraph and Sati to hide in Mobil Avenue (the Merovingian hates both the Oracle and the “prodigal son” Seraph).
Seraph probably deliberately avoided the backdoors since Smith obviously had access to them and guarded them. Smith guarded the backdoors to ambush Neo as he was trying to reach the Source, and Smith had previously ambushed Niobe in the backdoors after Niobe had just finished talking to the Oracle (seen in Enter the Matrix). Notably, Seraph opened the backdoors for Niobe from the door to the Oracle's apartment so Seraph clearly could have accessed the backdoors when he was trying to evade Smith. But he didn't since he knew copies of Smith would almost certainly be guarding the backdoors.
Ironically, the best place for Seraph and Sati to hide was an unimportant, abandoned apartment – which is where they did attempt to hide. Unfortunately, Smith went looking for them and found them anyway.
See the transcript for The Matrix Revolutions for the above quotes.