OP. What a horrible situation, for all involved.
Is your DP currently allowed any contact (like phone calls..) or has everything been halted and it’s complete radio silence?
I would agree with others here, your job will be to support your partner right now, and keep an open mind as to what may happen next. He will be missing them desperately and also intensely worried.
How is he preparing for the May court hearing? Does he have legal representation or is he representing himself? Ensure that whatever he submits is about the children. All language and dialogue must be about the children’s wellbeing. As an example the courts don’t care so much that a parent hasn’t seen the child, they do however care about the impact that has had on the child/ children. And what those children have been told.
I’m not sure how old the DC are but if they are in school, get a meeting with the teachers. Do they have any concerns? Have they noticed any changes in the children? Likewise, speak to the children’s GP, has anything been lodged with the GP’s regarding concerns with the DC?
Now, just gather info and support him. When the May court hearing happens and if contact is reinstated (and that is an if) , because a lot depends on why this contact has been stopped and the view the court takes on this...then I agree with others. Take a back seat and let your DP see how the DC are.
He needs to be clear on if the children ask why they haven’t seen him, he gives them answers that do not introduce conflict into their worlds. He cannot badmouth their mother. But he should absolutely talk to them about it, but only if they ask.
Your best job is just to remain neutral, clear space so you can both focus on play and something they enjoy. Be prepared that they may be withdrawn/ different, but just go with it and don’t say anything.
Good luck.