There are no contraindications in dealing with children in transmitting them Reiki (especially if they are very small). It is only necessary to listen to your heart and your instincts and trying to catch all the signals that children send to us.
Children have an energy still very pure and strong and accordingly treatments can be shorter, if the children are very young are sufficient even a few minutes.
Never force a child to undergo a Reiki treatment when you do not want and do not extend beyond the necessary treatment when he gives evidence of having received sufficient Reiki energy: Children are much more sensitive than adults and perceive with clarity when the treatment is enough.
For infants and children up to 3 years 5 minutes of treatment are more than enough.
From 3 to 5 years may be enough 10 minutes.
From 5 to 10 years could fit 10 to 20 minutes.
After 10 years you can further prolong the Reiki treatment. After 14-16 years old boys you can do one minute and a half to position (half the time compared to adult) or 2 minutes per view.
Towards the 16-18 years you run the Reiki complete with classical times.
As regards the possibility to introduce children to the Reiki there are drawbacks: you could become Reiki channel at any age.
In any case, a Reiki teacher should, in good conscience, be a person who begins making choices in personal autonomy, who has knowledge enough to accommodate the path of Reiki.
To start Reiki on minors, of course the parents have to agree. So it is advisable, as a reference, waiting until the child is at least 12-14 years old, as he could show a sincere conviction, in getting the first REIKI level.