D.M.
S.,
I'm a single Mom and it's tough to make friends when you've got SO much on your plate. Managing a personal life, and kids can be tough.
First, I would ask each kid what interests they have. Sports, dance, art or anything that can get them out of the house and mingling with peers. Don't assume that sticking them in an activity will be a cure all for their lack of socila life. Make sure it is something they WANT to do, otherwise you are going to have a WHOLE other issue to deal with.
Make some plans to take them places and plan family outings. Even if your friends don't have kids or have older kids include them in BBQ's and events. They just may need to see you participating to be encouraged to get out their and live a little.
Encourage them to be a part of things outside the house...find things that are age appropriate and ask if you can drive them and a friend. Suggesting things makes it much less like pushing them to do something.
I know after I split with my son's father, it was much easier to live in the house and just take care of things that needed to be done. But, getting out of the house with my son to play dates, the library and the market are super ways to jump start my day...
Good Luck.