In that case you will need to apply to court to get a child arrangements order to see your child. It's a civil matter so unless you think your child is in immediate danger, the police will not get involved.
To make an application to court you will first have to attend a MIAM (mediation information and assessment meeting) - see the form for more information about that. You can then complete a form C100, which the mediator will sign for you (a requirement before applying to court). Once the application is issued, the court will set a hearing date and ask CAFCASS to contact you and the mother to find out what the issues are - and do background checks with police and social services.
As you do not know where the children or their mother is, you will also need to complete form C4, which the court can use to order the DWP to give mother's address to you. If she is not working, she may well be claiming benefits. The form can also be used for the court to order anyone else who knows her whereabouts to give that information to the court - so if you have details of other people who may know her location, you need to provide those on the form.
The cost of the MIAM will vary according to which mediator you choose. The cost of the C100 and C4 to file is £215, unless you are on a low income or benefits, in which case you may be entitled to fee remission.
If you would like links to any of the forms I have referred to, let me know.
Does that answer your query?