It's easy if they are excited to get started, like the beginning of summer camp. Then you just say Go Get 'Em and smile and wave. It's hard if you get the This Was Such A Mistake I Wanna Go and you have to just give a solid hug, Oh Just Give it A Chance and send 'em back in. I don't think anyone has a good answer for that. Quite a few parents do the goodbyes at the airport and their swab travels alone (or I guess as a pair in your case.) Lots of stories on this site about it. Generally you can guess at the advice: know your family, do what seems best at the time, and then write a lot. The mail can be inconsistent but when it arrives it's gold. Know that there are huge swings in the mood and the tears in one letter might be miles behind by the time you hear about them, or the crushed/stressed/defeated kid on the phone could be fine in a day or two. Listen well, be supportive, a lot of the time the venting is therapeutic so you just need to let it flow and not freak out on the call.
For drop-off just try to keep it fun and forward-looking, like the party at the end of a big adventure rather than the culmination of years of meticulous planning and hard work that has to turn out perfectly. It's just one day out of 200 weeks, so See You Soon takes a little bit of the edge off and maybe makes it a little more mundane. Grind it out and we'll talk soon, You Got This, and then you can go to pieces in the car.