If you have not decided custody or it has not been decided in court then legally you both have equal access to the children. Except in a case where there is domestic violence…but even then she can not keep the children away from you.
My first suggestion is to make sure to keep records of what you are paying her in child support. Second, consult an attorney and get a separation agreement drawn up. Last, make sure to record some of these conversations somehow. Send her a letter or e-mail requesting information about the school and visitations. Let her know what days you will be picking the children up and be there. Record all attempts and refusals.
I’ve said this many times, but just because you lost your spouse does not meant that a child has to lose that parent. The parents are the ones that decide. If they want visitation they should be given every opportunity to do that and the other parent has no business trying to interfere with that. If they don’t want it then you can not force them to take it. Please consult an attorney.