I'm not a legal expert by any means. From what I can find about drug tests it doesn't seem that kratom would show up. I'll provide a link to the explanation. Also if he's been violent and you have it reported then surely that would factor in. I think it would take much more than him saying you take drugs. He would have to prove it and show how that made you unable to care for your child. I would say speak to your solicitor or even ask on one of the UK legal subreddits. I only know of r/legaladvi