I'm a single parent and obviously smee is more used to dada not being here all the time, but she does go through phases of asking for him alot. I'd say:

For the boys:

Talk about daddy, and have his picture handy. The crossing-off calendar idea is fabulous and will make a big difference. Have daddy bring back a gift (or buy a gift for when he comes back) by all means, but giving them a gift before he goes might be better - an activity they can do and think about daddy? Art or something, nothing too messy for you to deal with! When Smee asks for dada i say "not dada now, dada later/tomorrow/whenever" i never lie or distract as i find she gets upset if i'm vague. I think the important bit is to keep everyone aware that it is very temporary and daddy will be back soon so getting upset isn't necessary.

For you:

Make life as easy as you can. Cook extra and freeze so you don't have to cook every night. Get takeaway. Try to do the housework as you go, as it's so hard to catch up once you're behind - that's with one kid, i can only imagine with 2 it'd be far worse! Accept help when it's offered and ask for it if it's not. Be strict with their bedtime and make sure you get time every evening to put your feet up, arrange for adult company if you don't have any (does your MIL live with you?) so you can have a grown-up conversation once a day. Arrange at least one day with friends/mothers group/company with kids a week so you all have something to look forward to and so time passes fast.

Time will fly.

Bx