our springer spaniel (similar-ish temperaments to collies??) was our baby and 7 and a half when DC1 arrived on the scene.
She had grown up with my nephews and nieces and was really, really good with children - very chilled, had never growled when one of them had pulled her ears/fur/tail - so we weren't too concerned.
I think she knew what was coming, tbh, and my DH did take home a worn sleepsuit before we came back from the hospital so she could get a whiff of the new addition.
The transition was fine, and we just made sure that we paid her lots of attention and kept our dog walks up. Within a couple of months they were lying together on the playmat and I felt pretty much 100% happy that there wouldn't be a problem leaving them there even if I did go out of the room for a minute.
We didn't bother doing anything when DC2 arrived...so I think we just took her previous reaction for granted!
Unfortunately she died when DC2 was 6 months old, but have a new bouncy puppy that will have to get used to DC3, when they arrive. TBH, I'm more concerned now about DC2's reaction to the new baby than the dog's!!
Good luck, I'm sure you'll all be fine.