Dolly - at this stage, dont worry about access to the children, your youngest is too young to leave you for long anyway... Agree one day a week or whatever suits you for now.....
I went to mediation with STBXH as he kept quoting that he had equal rights over DD and that I was being awkward, yet he kept turning up late, changing plans the day before, bringing her back late. Quoted his equal rights, yet refused to have her a day a week in the holidays....
so do not worry about long term access at the moment, that is something that you can sort out as it goes along.. The main thing is to keep the kids out of it as much as possible.
It was so hard for me, with DD crying and saying I miss Daddy. It broke my heart to see her like it, but all I could do was to reassure her that we both loved her. The bastard sat her down and told her that he was going to live somewhere else and walked out. She tried to pack her little bag and go with him. I will never forgive him for that ever. So how you handle it now, is vitally important for your children. All they need to know is that you love them and that Daddy loves them and that they will still see Daddy.
I know how hard it is. I am so so sorry for you.