time4tea, occasionally dd 4.6 will say this, usually because she is tired and went to bed too late the previous night (we are working on that ...).
My response is as others have said:
Remind her of her friends and how they will miss her
What activities they would do today in nursery school
Ask her to choose something to bring in to show her teacher/friends. She can hide it in a secret place like her pockets.
Ask her to think about what she wants for breakfast/lunch (she goes to breakfast club and has hot lunch on site) and who she wants sitting next to her
Let her wear something special to school to show her friends, like a vest with Ariel on it or something, but within the boundaries of the school rules as she has to wear a uniform
Dd is quite extroverted and will generally respond to one of these things. I understand what you say about ds1 not being 'in' with the children yet, but it will come in time. 2 days a week is difficult to cement friendships.
If all else fails, I tell dd that even if she does not go to school, I have to go to work and if she wants to stay at home, she will be all by herself (a prison sentence for her. Thankfully she has not called my bluff - however, I do have a live-in aupair who can cover me at home, if I needed to get tough.
I sometimes say that it is the rules she must go in and I will need to write a note to the teacher to explain why she is not attending. But that usually backfires as she gets quite taken with the idea, lol.