ninathenana
Thank you for clarifying.
Dad doesn't have a say in that case, as long as the children aren't in any physical or emotional danger.
If his concerns come from things the children have said or their unhappiness, he should be trying to be positive with them but still able to address those concerns with mum.
If this is a newish breakup, it might just take a little time for all involved, both children and adults to adjust to new situations.
Be patient, it will hopefully pass