children's father haven't seen them since early December.

Did you get an access order with your maintenance order? If not sounds like you need one, if you have you need a new one! Yes get your solicitor to write to him you can't afford not to! If you still have no joy summons him yourself for an access hearing at your local district court. Afaik it's only 7euro. You'll be well able to represent yourself.

The judge will not be played by his manipulation and emotional blackmail. They will not tolerate him using his kids as pawns which is what he is doing. His choice will be simple - be an involved dad by seeing your children at reasonable and organised days ( typically every second weekend and a set weekday night but there is no rule) or pay extra maintenance so that you can organise childcare to work or a babysitter to socialise and get a break. Normally the parent getting access collects and drops back unless otherwise agreed so again you'll get a sympathetic hearing.
Put simply the maintenance he pays now is based on certain access which isn't happening and that isn't fair to you. You will get a very fair hearing as you wish to work while he has the kids. Ask your boss if they will give you a letter about your work arrangements.
Judge will issue a new access order based on what he/she hears. Your ex will be warned to stick to it or else he/she willbe iinviting yoi back to make a new maintenance application.

I've gone through the very same thing and my advice is probably going to get me attacked but here goes anyway !
My ex and I spilt up 4 years ago, he had been a fairly hands on Dad, never an amazing husband but a pretty good Dad. For the first year he took them every 2nd weekend, he wasnt working but wouldnt take them any other time (I too work weekends) Then over time, I had to bring them to him and collect them, despite him only living 5 min drive away. I argued to myself though the feelings of resentment that I had to put my kids first (then 10 and 12) for the same reasons given above. I then found out the kids were not getting adequate food at his, a bag of chips for dinner, beans on toast ect, so I began sending food with them. I hated this but I didnt want the kids feeling hungry. Then he began coming up with various excuses why he couldnt take them (usually last minute too) He never showed up to any school things and from the start never gave me any maintenance. I hated the whole thing as he often had girlfriends staying too, these girls changed on an almost weekly basis, which my kids hated ! Then 2 weeks before Christmas 2 years ago he dropped them at my place of work saying he had a "headache" and couldnt take them. I still had two hours of my shift to do and had to leave them in the staff toilets till I was finished work. I felt this was the last straw so I didnt ring or text him over Christmas and come late January he rang giving out to me about how hurt he was the kids hadnt rung him Christmas day !!
I told him that was up to him as the adult and they too were extremely hurt. My kids at this point were really struggling with feelings of insecurity and low self worth and the eldest I found out had started self harming. When I told him this he blamed me, and told me not to bring her to a counselor because they would twist things and blame him. He really couldnt see how his plan to hurt me was only affecting the kids. So I changed tact. I sent him a nice email, explaining that since I was working and he wasnt, and that because he wasnt going to help I was going to hire someone. This would involve me taking on more hours at work so if he wanted to see them he would need to collect and drop them off. I would no longer be sending food down to his with the kids as again this would be need to cover child care costs. We never heard from him again.

It was really hard at first, the kids were very upset but I was very lucky that their schools both had support programmes (Rainbow in primary and a counselor in the secondary school) I also applied for FIS payments which helped financially. Over the last three years together we have worked through a lot of negative feelings and are now coming out the other side. And I am amazed to have heard from man other women that they ended up doing the same thing. Despite what conventional wisdom about co-parenting might say.
I never kept the kids from him, I just left it in his hands plus I have said to the girls that perhaps when they are older they might want to reach out to him but both are adamant they want nothing to do with him. I honestly think this was the best action I could have taken.
One of my closest friends has gone through the same thing and she kept trying to to co-parent, her kids are still upset and angry, their life is still chaotic with last min let downs, fights over money and all the rest. While our lives are far from perfect, I'm broke all the time, I have pretty much zero social life and I badly need a holiday ! Over all our lives are calm, the kids are happy and stress is at a minimum. I say send him a similar email, call his bluff and get rid of him that way. He's never going to support you or put his children first. Make piece with the idea that you are a single parent in every sense of the word and get on with your life. Perhaps an Au pair might suit you ?

Any doubts I have that this was the wrong road to take were put to bed recently. I had to contact him to find out were he was living so I could finally apply for a divorce. In the 5 min conversation he never asked about the kids, simply saying as he rang off "give the girls my regards" it was a cold and empty platitude, meaningless words. So there you go, its not fair and its not right. Your going to have a lot of hard work ahead of you with very little support but if you stop fighting, instead accept the pile of manure that is your life now, and in a year or two, roses will begin to bloom ;-)