1. The Convenient Friend

WHERE TO FIND HER: Look around. She's your neighbor's kid or the toddler with the friendly mom.

WHEN SHE'S GOLDEN: Post–blob stage until around 5

INVITE HER OVER BECAUSE: This first pal is the one who teaches your child that friends are fun.

TO MAKE IT WORK: Yes, playdates are often like an arranged marriage, but they can succeed. You need to:

• Keep it age-appropriate. Toddlers don't interact the way older kids do, so don't badger them to toss a ball. Give them a safe place to play and let them be, even if they ignore each other.

• Lay down some simple rules. When my kids were younger, I always recited the house rules for friends as soon as they entered the house: No hitting. Share the toys. And if a friend whacked my kid with a Duplo? I diverted them with a toy, and tried not to hold a grudge.

• Use proximity to your advantage. Lauren Wu was thrilled when her daughter Maddie, now 7, hit it off with her neighbor's son. "The kids could get together at nontraditional hours that worked for us, like early Sunday morning," says the San Francisco mom.

• Know when to call it a day. For toddlers, an hour or so of togetherness is plenty (preschoolers can go longer).

2. The Opposite-Sex Friend

WHERE TO FIND HER: Anywhere boys and girls mingle -- playgroups, music class, library storytime

WHEN SHE'S GOLDEN: Babyhood to 5

INVITE HER OVER BECAUSE: You don't want your son growing up thinking girls are aliens from another world (or vice versa)

TO MAKE IT WORK: Forget stereotypes -- boys and girls can be friends (really!). Instead, try to:

• Recognize what's normal. At 2 or 3, gender doesn't make much difference; by 4, however, kids prefer same-sex friends. So start early, says Michele Borba, author of 12 Simple Secrets Real Moms Know. "Children with opposite-sex friends develop more respect and empathy for the other gender," she says. "In the long run, they'll get along better in life."

• Paint it positive. Lisa Chinnery's son James, 3, is the only boy in his preschool one morning a week, but his mom focuses on the fact that he's often the center of attention. And being with his female classmates has expanded his horizons: "He's much more interested in make-believe and stuffed animals as well as his usual cars and trucks," she says.

• Watch your own messages. Kids pick up on verbal and nonverbal cues from you. So curb the "Only boys are so wild" and "Coloring is better for girls" talk, says Borba. "You have the ability to break stereotypes from the beginning," she adds.

• Nix your inner matchmaker. They're not headed to the prom, so squelch the cutesy impulse to pair up preschoolers or refer to them as "boyfriend and girlfriend." Such talk will only make them say "Eww!" to being friends.