Or HaChaim on Exodus 11:8/אור החיים על שמות י״א:ח׳

1 וירדו כל עבדיך, "and all your servants will come down, etc." This was a promise by G'd that the servants referred to would all survive this plague. Perhaps this was because none of them were firstborns and Moses knew this through his prophetic vision. Alternatively, even though some of these servants may have been firstborns, Moses exempted them from the effects of this plague. This was not because of their righteousness but so that they would get their deserts for having been present and silent when Pharaoh used abusive language against Moses and Aaron. The principle of the punishment fitting the crime demanded that these servants would themselves be demeaned when they would beg Moses and Aaron to leave the land in order that not all the Egyptians would die. They would have to ask public forgiveness for their previous conduct. We know that Moses had felt insulted by their behaviour since the Torah told us at the end of this verse that Moses left the palace in a very angry frame of mind. Actually, Moses included Pharaoh himself in his warning that all his servants would get up at night, etc.; he only maintained a degree of respect for Royalty and that is why he did not refer to Pharaoh by name.

2והשתחוו לי לאמור, "they will bow down to me saying, etc." The reason that Moses said לאמור instead of ויאמרו לי, was that he implied that the mere fact they would prostrate themselves would convey their intent to beg Moses to leave the country with his people. Inasmuch as Moses was superior to them why would he need their permission to leave altogether?

3In addition the Torah tells us that the servants treated Moses like a king. Just as it is not customary for a citizen to approach the king and begin to speak without having received permission, Pharaoh's servants prostrated themselves as a sign that they asked for permission to speak. The Torah makes this plain by quoting Moses as saying: "they will bow down to me to say, etc;" the bowing was a form of asking permission to speak. While it is true that the Torah does not report these details as having occurred when the events of that night are described in chapter twelve, the fact that the Torah does report that Pharaoh himself arose during that night is sufficient proof that whatever Moses had predicted did in fact occur.