It dawned on me about a year or so ago that my oldest (he was about one and a half at the time) was starting to be at an age when he really would benefit from having some friends. I hadn't gone out of my way to set up play dates or even take him places where there would be a lot of other children around. I blamed it on the fact that we moved around so much (nine times in four years).I've also leaned on the "I'm shy" crutch for a lot of years, so I hadn't generally made a habit of making friends every time we moved. I'd mostly keep to myself or hang out with one or two other people who had moved with us (my husband did summer sales). But it was getting hard to deny that my son needed friends and that it was up to me to help him make that happen.
Take them where other children will be. A good way to start, I thought, was to take Lincoln (my son) to the playground or to a children's museum where he could run around and make friends. This was effective because, luckily, Lincoln is extremely friendly. He makes friends wherever he goes, as do most small children. They are amazing at making friends! It's like they have a little-person radar and they can seek out the children who are their own age and make immediate best friends with them everywhere they go.
