Felting is tricky. The main things which affect felting is the temperature of your hot water, and how much agitation you have in the washing machine.
Here's what I recommend: knit a swatch from the yarn you plan to felt. Then, set your washer to the hottest water you can get, add something to agitate it, and time how long it takes for your knitting to felt. That will tell you how long your WIP will take.
I don't have kids in my house, so I keep my water super-hot. I also use clean tennis balls for agitation, so I can often get an item felted in less than 30 minutes.
The great thing about felting-- you can keep washing the slippers until they get to the correct size. Just remember, it's the heat of the water PLUS the agitation which gives you felt.
Good luck, I'm sure you'll be able to get the slippers down to the correct size!