Either some powdered colostrum, or, better still, some colostrum from a neighbouring cow in your freezer.
I only have 2 cows but I keep a stomach tube kit just in case calfie can't suck (and is born at 1am...)
Calfie needs to get some colostrum within 2-4 hours of birth, and if Mum can't or won't play ball, you need to be able to step in. If calfie can't or won't suck, use the stomach tube.
You could probably get away with using a lamb anti-vac feeding bottle if you had to, and buy a proper calf bottle when the shops are open if Mum hasn't been able to take over by then.
Mum could be very dangerous when calving or just calved, so make sure you have a penning arrangement where you can keep yourselves safe and get to the calf if need be.
Rope halter for mum. Even if she isn't trained to it, you may have to lassoo her and tie her up to get calfie onto the teat. (It's rare with Dexters, especially an experienced cow, but sometimes mineral or other deficiencies can show up as a calf - or lamb - being very half-hearted about feeding.)
Almost certainly you will need none of it, but it's so much better to have it and not need it than the opposite!