Depends on how you define "child", I have a 15 year old niece who would take great offence if you called her a child. Probably worth bearing in mind that the age of consent in large parts of Europe is 15, or even younger 14 in Germany.
It’s not as cut and dry as that. In Germany there’s the ‘sexual self determination’ rule. Under that Johnson wound still have been charged with the same offenses.
It’s irrelevant anyway, he knew it was an offense in the UK and he still went ahead and committed the offenses.