J.M.
First of all, if you need to cry at work, go ahead. I wouldn't weep openly at your desk, but if you need to take a few minutes for yourself in the ladies room, do it. I'm sure that if you work with reasonably caring people they will understand that the first day back is a tough one. Some practical things I do when I'm feeling bad about being at work:
1) Get really busy with work stuff. Focus on the task at hand and busy my mind and time goes faster.
2) Remind myself that my kids are in a great place, safe, having fun.
3) Remind myself that most dads go off to work, that we expect them too, and that their kids still love them very much at the end of the day.
4) Be honest with myself that even though I miss my kids very much in the moment, I wouldn't want to be home all the time.
5) When I drop my kids off, I make sure to tell them "you're going to have so much fun today! You're at the funnest place (or with the best person)!" etc etc. I don't tell them that I wish I were going to be with them or I'm so sad that I'm leaving them. I think it makes it better for all of us (even the baby). Then when I leave I believe that they're really going to have a great day, and so do they. My kids have never really been super sad at drop off, which makes leaving a million times easier.
6) Sometimes I call the daycare. But I only do this if I am 99% sure that they're going to be doing great (not crying, or overtired) because calling to find out that they're sad is really terrible. But calling to find out that they're having a great day is wonderful!
Good luck. The first couple of weeks are a little rough, but you will get in to a groove and things will go great.