Absent parent causing emotional damage?

My two children 6 and 8 wish to spend more time with their dad
He has them once a week Friday 6pm to sat pm one week and sat 6pm to sun 6pm alternate weekends

This is how much time the ex wanted though to me he''s welcome to have them as much as he wants.

The children know I''m happy for them to go whenever but dad does not budge on extra time.

8 year old has phoned dad to ask him if the can spend the weekend with him and dad said no I''m going out.
Son sobbing his heart out and very angry and dad says well last time you where really naughty I''m not going to spend more time with you if you can''t behave.
I cannot believe he would say such a thing! Basically blaming an 8 year olds behaviour for him not spending more time with him.

One- I don''t really want my 8 year old ringing him and asking him but ex will not talk to me,so much so if he sees that I''m calling him he ignores it so the kids leave an answer phone message and he rings them back just so he knows it was the kids calling and not me.

Two -last time I tried to speak to him face to face because my oldest child told me daddy shouts at him for asking to see him more he told my son if you don''t get in the car now I''m leaving you. Then tried to drive off with the car door open and my son not even buckled in.
I now don''t dare try to speak to my ex because he would put them in danger just to avoid having to talk to me.

I''m in bits i have no control over this situation but I''m so scared of withholding access and being labelled one of those kind of parents.
I don''t want to cause more harm to my children as I really want them to have a relationship with their dad.
I have requested to go back to mediation but he''s not paying for such crap again apparently

What are my rights as parent with care? Can I ask anyone to intervenes to access his parenting capability
I really don''t want to reduce the time the children have with him as they miss him so much but I''m so scared he''s messing them up.

As there is a routine in place, I wouldnt be allowing the children to ring for more time, its not acceptable to place the children in the middle. You know he will say no and it will hurt your son.

Your childrens emotional welfare will be more impacted by not seeing dad and you know the impact it has on your partner not seeing his child.

Thanks for the reply
I''ve emailed the ex this morning stating though it''s part of the arrangement the children will not to phoning anymore.
All it does us upset our son and he''s obviously not old enough to have to speak to you midweek as it upsets him.
If he wishes to spend more time with the children he''s to contact me direct regardless how much he dislikes talking to me.

Only thing is my ex will go back to telling the kids mummy doesn''t allow him to have extra time like he did when they where little
It made the oldest hate me for a while because my ex was using me as an excuse not to see them when I never said such a thing.

If he is complying with the initial verbal agreement, well that is fare enough .
Think it be very unwise of me to comment on anything else. In an ideal world it be great if these situations worked well post divorce sadly the reality is they do not. Hopefully in time a resolution will be sought which will make the situation better for all.