As a homeschooling mom, my goal for socializing my children is to encourage them to interact appropriately around other members of society. I want them to know social norms, and how to pick up on social cues. I don't limit this to interactions with other children, though they each certainly have friends they enjoy playing with.
Here are some ways that we work on socialization, or opportunities to use our social skills, and times where I may take a few moments to offer instruction/guidance on those skills:
-While shopping--looking for opportunities to help others, being kind, courteous, using inside voices, etc.
-Taking swim lessons
-At random parks
-In waiting rooms
-Volunteering at long-term care
-At church events
-At the fair
-While playing with a friend
-At story time
-At the museum--both in a guided tour setting, and while exploring
Playing together, as siblings, is also an excellent way to practice socialization skills. Your children can learn conflict resolution, sharing, playing kindly, and so many other essential skills just in their normal day-day activities.
You don't have to constantly be seeking socialization for your children. Use the natural events that occur in your life, and look for teachable moments to increase social skills.
As they get older, have them call and schedule an appointment or two. Teach them how to answer the phone respectfully, how to purchase items at the store, how to ask for help if they need it in the library.
People are everywhere--teach your children how to behave around them, wherever you are.