Congratulations on four months! That's terrific. I have no experience in salary negotiations. Terrible at it and where I work had structured salaries when I began.
But, I do have experience with daycare issues. If you are contemplating a return to work, shop around to find out where you may place your children and find out the best fit and what the going rate might be. If you are seeking a private person, find out what they might charge. If it's a center, find out what their rates may be. Find out if they are flexible in hours.
If you will have set hours with the company you will start with, make sure they are family friendly because if one or both of your children becomes sick and can't go to daycare you will need to be able to call in to care for them or else make sure you have back up.
This isn't on point on what you asked but it's important. I suddenly and recently had to deal with both issues - finding day care at reasonable rates and having two sick children. Mine are in school full day but I needed someone to pick them up from school and watch them until I got home from work. They were sick over their spring break and my mother graciously watched them as we are so short staffed that I could not possibly call in to care for them. It was highly stressful to manage.
Good luck with your negotiations. I second Non's advice to go in first and go in higher. Also calculate any fees or costs you would not otherwise be paying except now you're working. Good luck!