I agree about looking in places you think she can't fit. Look in every cabinet, behind things, places you looked that might have had a place she could hide you didn't think of.
I once looked for my Tarzen for hours. In a 1-bedroom apt. I thought maybe somehow he had gotten out because I really looked everywhere. When he decided I should find him, he came out.
He was sitting on the windowsill. which was one of hte first places I looked. I had moved the curtains aside, and didn't see him. He was sitting right in the corner of it so I moved the curtains aside right over him. He must have thought it was hysterical.
Its possible she did go to have her kittens. Because she is so big and might have so many, you really shouldn't wait in trying to find her. What if she needs help?
Maybe you can make a game of it with the kids to keep them quiet and they can help. Or offer a reward to the first who finds her.
Or everyone sit each at a doorway of a room, make it very quiet, and see who might hear something first.
Maybe one of the kids opened a cabinet and she got in there and behind something, maybe where you keep your towels. Or a toybox that was open and one of the kids closed. Maybe in a vent.
When my foster, Snowflake was here the first few days, she disappeared. Guess where I found her. She had gotten behind the refrigerator, and there is an opening into where the motor is. She was actually inside the back part of the fridge. You wouldn't think a cat that size would fit there. There are holes like that all over your place you forgot about. So look not only where yo think she is, but where you think she isn't.
Also, if you can get all quiet, sit in the middle of the house, and just meow to her, then wait. Maybe she'll answer at some point to give you an idea of where the sound is coming from.