Is there a word in English commonly used to mean "penis" when speaking to babies or children?
For example, in Russian we say "пипка." My son's learning English from me and I want to teach him this in English.
The reality is that this is very much an opinion. You might find some interesting statements on the Parenting site but, personally, I believe that kids should learn the correct terms for all parts of their bodies, including genitals. If they don't grow up using these terms, they are likely to feel embarrassed using them as adults. The easiest way to avoid this is to start them out using the terms from the beginning.
One of the links to articles that Damkerng T. posted in the comments is useful.
Many moms feel very strongly that only the anatomically correct names for private parts should be taught to a child. As Angie B. says, "It's no different than teaching him fingers, toes, nose, and ears. They are what they are and there should be no shame in using the appropriate names." That means teaching toddlers the words penis, testicles, vagina, and vulva.
Many moms also say that in addition to causing children to feel embarrassed or ashamed about their private parts, nicknames can cause confusion in communications between the child and their doctors, caregivers, and teachers, should any serious issues arise.
The American Academy of Pediatrics agrees: "It is important to teach your child the proper names for body parts. Making up names for body parts may give the idea that there is something bad about the proper name."
If you choose not to subscribe to this feeling, perhaps because of your own discomfort, there are other options listed in the article. Some that I can think of:
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