Ah, poor you - 12 days is a long time.
I travelled most weeks for work, away 2-3 nights, from my daughter being 13 months. She's now 6 and I do 5 nights away in a row every other week. Occasionally a full 7 nights. And not work related, she did 15 nights away from me with her father in the summer (now divorced).
Firstly: we have a lovely bond. She's the same with me as my friends kids are with them - cuddly, affectionate, not clingy, happy, seems secure. Lots of children have parents who work away, are divorced, or do shift work. If you'll be away regularly, don't panic.
Secondly: we have never called or skyped. I say never... When she was 3 they called as she'd had a meltdown over being told to put a coat on and now XH couldn't handle it 
But we had decided that as she was to begin with very young, talking would upset her. That's worked well now we're divorced. At 6 we can chat about everything, so she knows now that she can always say she wants to call the other parent. She never asks.
I do think 12 days is a long time for 3 & 4 years not to speak to you. BUT - my experience of Skype at that age is that it was rubbish, they wander off! I'm more a fan of a planned bedtime story call. Also think the morning call suggestion is a good one.
Most important thing - your husband has to be honest and clear with the kids. If he says "we can call mummy if you like, but it's fine if we're busy playing - I'll tell her we were - now, shall we make her a welcome home card?" then they will NOT be damaged. Yeah they'll miss you, might be clingy when you're back for a bit... But they'll simply lesrn that it's true mummy always comes back.
I try to always be positive... I never say I missed you when I come home. Only "baby I'm excited to see you again!".
You'll be OK, they'll be OK. Absolutely a family decision how to handle this though, not his alone. xx