Although tasers have been drawn 22 times since 2016 in the presence of children (under 18), they have only been aimed 7 times and have never been discharged.

https://www.scotland.police.uk/assets/pdf/434027/599268/599276/20-0236-response?view=Standard

Data from England is not easily identified but the Children’s Rights Alliance for England estimated that tasers were fired on children around 25 times in 2013 alone:

http://www.crae.org.uk/news/increase-in-use-of-tasers-on-children/