Paul S. sent in a link to a cell phone for five-year-olds (plus or minus). The fact that it has a button for dialing a female and a male person (presumably mom and dad) is a nice illustration of how having two parents of opposite sexes is normative. Many children have two caregivers of the same sex (e.g., a mom and a grandmother or gay parents) or only one caregiver (or, for that matter, more than two). But it is expected that a child will have a mother and a father involved in their life. That this is “normal” will be reflected back at that child again and again, whether or not it reflects their reality.
For more examples of heteronormativity see this post (featuring sea monkeys!) and also see our post full of cute photos of same-sex animal couples.