Question on the tablet.

Hey it's me again! lol but it is kind of stated in the third film, more like the history of the tablet is explained which pretty much gives the answer to your question. I don't want to spoil it for you though. So I'll post it but it'll be in the spoiler thing so you can choose to read it or not :)

In the third film Pharaoh Merenkhare explained the history behind the tablet and he basically said that he didn't want to be parted from his his family even in death. It was obvious that he deeply loved Ahkmenrah, so he had the Tablet of Ahkmenrah created and blessed by a Moon Priest so that whoever dies, will be brought to life at night, considering it was created by the light of the moon. This is why it only works at night. It has the power of the Moon God to reanimate.

Even though they kind of show in the beginning of the third, we can assume that before Ahkmenrah was sent to Cambridge and his parents to the British Museum (It's up for debate whether or not Kahmunrah was truly sent to the Smithsonian or if that was just a wax replica considering the opening of the third one) that they were able to wake up in their tomb, as they were tombed together and be together until they were excavated and separated.