Help with children visitation. England
So long story my wife has three children with her ex husband. I’m her current husband for context, in February our oldest had a meltdown one night kicking and screaming so we suggested for the wellbeing of the younger two children that she go’s to stay with her dad for a few weeks until she had calmed down. Now this has carried on and she’s not wanted to stay at our house. Which has been fine everyone has been a lot more relaxed since this. Now comes the problems after 3 weeks of being at her dads he claimed child benefits for her which we didn’t expect her to stay that long but it’s ok well now the other problem. He’s claimed child maintenance off my wife but did it very sneaky he used her old married name and a phone number she’s not used in 8 years. So when we found the letter 4 weeks late. (wife is terrible and checking her mail) she calls up child maintenance they advised her to put a counter claim in as he hasn’t given any money for the kids in nearly 3 years. Well he got the letter today and has called up my wife kicking off shouting that they have them more than they are here and he’s claiming he wants sole custody of the younger two children. The kids were supposed to be going today but we don’t think he would return them so have kept them at home until we speak to a lawyer. What chances has he of winning as my wife is in bits about it. Thank you for all your advice in advance
Edit
Kids are 15,12 and 9. Dad has only demanded sole custody as a retaliation because we have claimed against him from advisement from child maintenance. Oldest has issues and possible underlying adhd and autism so can be extremely difficult with her behaviour.