First of all you've done brilliantly with the locks and setting up your own account, finances etc.
I cannot stress enough how well you have done. So many of us fall apart when told a relationship is over, we are 'walked out on' and so on, but you, despite your feelings of being trapped and isolated, have got on and dealt with the practicalities in an admirable way.
First of all you need to stop facilitating your six year old spending time with your ex. She doesn't want to, and that is not your problem, it is his. You need to make this clear to him in no uncertain terms. Her reluctance has nothing to do with you but everything to do with him and what he has done. Your role, after he has left you and his children, walked out on you - is not to facilitate his access - it is to make yours and your children's well-being paramount, and unfortunately for your ex, that doesn't necessarily involve catering to his whims, needs and requirements.
He is paying maintenance for the children because he has to. If he stops doing it voluntarily then the law will determine that he must continue to do it - so don't give him any Brownie points for that.
Seeing a solicitor is of course a priority - to protect yourself and your children - but there is another aspect to this, and that is your mindset.
Your husband has rejected you, your children and his life within your family unit. There may well be another woman (there usually is) but it seems from what you have written that you are already prepared for that.
Whether there is nor not, the point is that he has rejected/discarded you - and you have three choices here - to wallow, survive, or thrive. Most of us, once we get over the trauma of separation and divorce tend to veer towards the 'thrive' option.
From what you've written in your OP I think you are also of the 'thrive' persuasion. It's just a case of getting over the shock and sense of rejection, gathering your senses, and marshaling a plan of action.
See that solicitor as soon as you can and stop facilitating your ex. His relationship with his kids is his concern - not yours. If little one doesn't want to see him, then tell him straight. He cannot use his child maintenance as a bargaining tool - he will be obliged to pay that anyway.
A lot of threads on MS are very upsetting, but there is something about the way you write and what you say that tells me you are going to be just fine OP. A few months down the line and I think you will have a new lease of life, and be the strong one ... while things for your 'ex' with his new pad, bank account and fancy free lifestyle ... might not pan out quite as he hoped.