When the CPS come to inspect your house they will be evaluating many things, some of which are:
* Enough space for child
Is there a bedroom, bed, and places to put clothes and other belongings? Are there places to play? Do homework? Entertainment?
* Clean, orderly
While it might not make a difference ultimately, they do note whether the kitchen is clean (ie, they'll note if there are dirty dishes in the sink, for instance). While they don't expect perfect cleanliness and no clutter, they are interested in making sure that there's no obvious problems that might lead to illness. Adequate facilities for using the restroom, washing, etc.
* Safe
Dangerous items and chemicals must be properly secured, even when you're taking in teenagers, but especially when you're taking in children under 10. The area and neighborhood are safe. The house is structurally sound and safe.
* Food, shelter, financial security
They'll probably want to see your income tax forms to understand whether you have sufficient means to take care of the children. They may require information about your financial status including bank account balances and debts, loans, mortgages. They'll figure out what you generally eat, where, how you prepare your food. They'll go over insurance information, and inquire as to your life insurance policies (ie, how will you take care of your kids if your spouse dies, and vice versa). Whether you have a support system that will help you (family, friends, strong religious or other group affiliation)
There is a lot of advice online regarding adoption, especially adopting newborns, and while not all the same information applies, reading through it will help you understand what their goals and priorities are. They may not be what you assume they should be...
Added at: 13:12
In other words, pretend the children already live with you, or that if they were to appear tonight you would have everything all set to go. You may have months before things happen, but when the inspection occurs you need to be as ready to take them in as you will be when they actually do appear. Also, keep your house clean and uncluttered.
Added at: 13:20
Working window locks and door locks. If you have animals, do the animals pose any danger (ie, even if it's the kindest, gentlest pit bull in the world, they may not look on it favorably, but a lab is probably alright).
Oh, and your schedule - who is going to be home with the kids, or if you are nearby and reachable if the kids are old enough to be left alone.