Why not take Watson's Little Albert as an example. It is now considered unethical to evoke reactions of fear in humans under laboratory circumstances, except when the participant has given informed consent to being purposely horrified as part of the experiment. The experiments should not cause the human participants to suffer unnecessary distress or to be in any way physically harmed and this is especially true with children.

Watson, J.B. and Rayner, R. (1920). Conditioned emotional reactions. Journal of Experimental Psychology, 3, 1, pp. 1–14.

Beck, H. P., Levinson, S., & Irons, G. (2009). "Finding Little Albert: A journey to John B. Watson's infant laboratory". American Psychologist, 64, 7: 605–614.

Conditioning in early childhood is more easy than the others. They are easily conditioned to their moms actions, you can observe this phenomenon easily by looking at a baby. But as I know, there is just Watson's experiment, little Albert. I do not believe there is not any after 1950s because of ethical rules published by APA.

You might find Applied Behaviour Analysis offers some ideas. Professor Michael Keenan is probably the UK expert in its theory and application. He contributes to Researchgate. He is a member of staff at the University of Ulster @ Coleraine, Northern Ireland UK. Why not contact him? I am sure he would be only too pleased to help.

Sure. You'll find a lot of them typing "operant conditioning in infants" in "scholar.google.com". I suggest a brazilian researcher: Gil, MSCA

 Conditioned responses, such as head turning, to one stimulus or another in order to hear or see a reward, OR  increased sucking  (using  a special pacifier   equipped with special instruments ) are conditioned responses  used to assess infant attentiveness  ,for example,  for discrimination  of  different phonemes . Infants tend to habituate to the same stimulus after awhile.  So when a new phoneme is sounded, then the infant might start non nutritive sucking vigorously again in order to keep this new stimulus sounding,, and this tells the experimenter that the baby has indeed discriminated the new sound. from a previously presented one.

Thank you all - this is exactly what I was looking for.