Good advice from PPs above to ask police to do a welfare check. Make sure you tell them he has mental health issues, and that your DD is only 3. If you have any other specific concerns, make sure you mention those too.
I'm not a family lawyer, but this is what I would recommend if you don't have a solicitor you can talk to who knows your case:
You send him an email (not just a text - so you can use a copy as evidence if needs be), saying that if he fails to return her tomorrow he will be in breach of the court order - unless you agree with him. Tell him you want to assess the situation yourself - as she ought to be in your care from tomorrow, and since you have parental responsibility you should be involved in decisions about her medical care. Tel him that if he refuses or ignores you, you will not hesitate to take appropriate steps that are open to you - including via the legal system if necessary. Ask him to send you some photos of the rash immediately - by email or by text message. And ask him to let you speak to your DD immediately. Ask him what treatment he has been giving her (if she is so ill he has presumably been doing something), and whether he has contacted a doctor, and if so the name/address of the doctor.
If you get no reasonable response, seek an emergency court order showing them your email, and the lack of reasonable reply. Even if it takes awhile to get action from the court over the christmas/new year period it will help to have it on record, so if he does it again he will get into more serious trouble.
Keep your emails calm and business-like, even though you are probably very upset. Imagine a judge reading them.