My usual answer is two-fold -- I bet the idea is already there, and why not?
The idea is already there
ALL adopted kids think about their birth parents. If you've told them they are adopted, and explained that that means they had a family before yours, they are thinking of that first family. I don't know what they are thinking about that family, but I'm willing to guarantee that by age 4 (if not earlier) they ARE thinking about that family. That's what the research says. And yes, I said ALL adopted kids!
Once they're between the ages of 7 & 12, they are thinking A LOT about that first family. For some kids, they are mildly curious and for others they are obsessively curious. Some kids are going to be avoidant -- NOT wanting to think about or know anything about birth family. But guess what? NOT thinking about birth family in this way is thinking about birth family! [DON'T think about the elephant in the living room!]
Here's what it says in Being Adopted: The Lifelong Search for Self:
We are often asked, "What percent of adoptees search for their birth parents?" And our answer surprises people: "One hundred percent." In our experience, all adoptees engage in a search process. It may not be a literal search, but it is a meaningful search nonetheless. It begins when the child first asks, "Why did ithappen?" "Who are they?" "Where are they now?" These questions may be asked out loud, or they may constitute a more private form of searching -- questions that are examined only in the solitute of self-reflection. This universal search begins during the early school years, prompted by the child's growing awareness of adoption issues.
So the idea is there -- what are you going to do about it?!
Why don't we want to plant ideas about adoption/birth family in our kids' heads? We're ALWAYS planting ideas in our kids heads -- God loves you, math is fun, honesty is the best policy, be kind to others, recycle, whatever! We're parents, planting ideas in our kids' heads is what we do!
Do you know one single parent who'd say, "I'm going to wait until she's an adult to suggest she never steal -- I don't want to put ideas in her head!" Part of a parent's job -- perhaps the most important part of a parent's job -- is to pass on our values to our kids. Why is adoption exempt from this rule?
Now, sometimes when parents say, "I don't want to put ideas in her head," they mean, "I don't want to cram adoption down her throat." Great! I don't want you to do that, either! But there is a great divide between waiting for her to mention it to show that the idea is already planted through no fault of your own, and harping on adoption all the time. And the danger of waiting and doing no planting of our own is that our child will get the ideas elsewhere, from less informed and caring people (like kids on the playground -- "Your real mother didn't love you, that's why she dumped you like garbage!"); or they'll get the idea that you don't want to talk about it, so they'll seek out information from someone else, which means that SOMEONE ELSE will be passing on THEIR values to your child, not your values. And if they can't talk to you about it, it becomes this THING that stands between you, impairing your relationship. There was an IMMEDIATE improvement of our relationship when Zoe and I started talking about the hard things in adoption.
Sometimes parents say, "I don't want to put ideas in her head," when it comes to FEELINGS about adoption and birth family. Parents will willingly share facts about adoption/birth family, but don't want to suggest any particular emotions that might be associated with it for fear of convincing a child to feel a particular way. So you'll say, "You have a birth mother," but won't say, "Do you sometimes miss your birth mother?"
Again, I have two answers. First, exactly how malleable is your child?! I have a dickens of a time convincing mine to feel a particular way! You've seen the video of Maya saying she's not at all interested in knowing about her birth parents, and you have some idea of how much we talk about adoption and birth parents, including feelings and thoughts, in this house! Second, is that how you convey values about other things? "There is a God." Next! Of course not! We tell our kids how they should feel about things -- "Love God" -- and why they should do certain things -- "Recycle to save the Earth." We even tell them how other people feel, even when we don't know -- "Your teacher doesn't hate you!" Why, then is adoption/birth family off-limits? It shouldn't be. It's something that will always be enormously influential in your child's life, in your life, in the life of your whole family. No, it doesn't define us, but it will always be there.
So plant some seeds. And be amazed at what grows!