I don't think your mum is being unreasonnable. 7 or 7.30pm is actually a pretty reaonnable time for everyone to have a meal together, even those with small kids.
But, here what you could do:
1) graciously decline the invite and say that it is too late for you because of your children.
2) your and your partner go, with the kids, first signs of tantrum, leave! but bring plenty of things to keep her occupied and food. Kids love finger foods when out.
3) get your bro on the phone and invite and his gf to your home for another time.
4) your partner stays home and look after the kids. He can always meet gf another time, ie, christmas party.
5) get GF's phone number and text her and invite her for coffee.
6) Get the name of the restaurant they are going to and send a bouquet of flowers for gf. She will appreciate it, even if you can't be there.
Believe me, I understand the feeling of not being able to be there (If you choose to) and generally not being able to go to some invites. I can sense your disappointment and while you are not being unreasonable in your thinking, you are being unreasonable in your attitude as you said: ''am I being unreasonable to expect some consideration?''
I would say ''Yes, a bit'' because consideration would mean your mum hasn't thought of your situation at all. Maybe she did, maybe she didn't but it seems to me you are taking things personally and it does not need to be this way.
At the end of the day, you chose to have children and that means some sacrifices from both you and your partner. Not the others around you.
I will say that in the nicest way possible, but I think you may be the one making things difficult, there are so many ways to go about this.
Again, this may be because you are disappointed but at the end of the day, it is only a meal and meeting somone who you can meet another time.
If it was me, I would accept the invite and go and see how it goes. I assume you have a buggy for your daughter. If she gets tired, she wants to nap, put her in the buggy. Talk to your daughter before going and tell her you expect her to behave. Put boundaries, you are the mum. Explain if she is tired, she can go in the buggy. Explain that if she makes a fuss, then, that new toy she wanted, well, she won't get it (this is an example, whatever can work for you).
In the end, if you decide not to go, then perhaps you can one of my suggestions above.
By the way, is your mum paying for it all or do you all share the bill?
All the best.