Tfish - you aren't failing at GAL; this looks very different with young children in tow. Really, as the primary care giver to young kids, you still have to meet many of their needs to help then along. And, given all that is happening, you have to be present for them. However, I am sure you are burning out. Can your sister come and watch a movie with the kids so you can get out for a bit? Go do something nice for you?
As for the questions your kids are asking about their dad, you may want to come up with some sort of strategy here. You're walking a tough line because you want to protect your kids from what is happening but you may not want to act like everything he is doing is perfectly fine?
So when my h checked out and went MIA in replay, I started as you did: making excuses. But replay can go on for some time and the excuses may wear thin? I can tell you some of the things that worked for me/us and maybe some of them can help you if you want to try different things.
I co-slept with my kids for quite some time when my h started acting off and the kids started to react. This reduced their anxiety. I showed lots of affection: hugs for no reason, hair touslling, etc. I listened a lot and I answered my older son's questions honestly. Both my kids knew their dad was off and I started to say "your dad is going through a funk. It happens." And I shrugged it off. They followed my lead.
If your h is communicating through you as to when he will come, you may want to consider saying nothing to the kids so as not to set their expectations? Maybe you already do this? If he communicates with the kids and then doesn't post, this is where it just is so tough. This is where I started to say "he sure is in a funk." "People can be off. It happens." Then I would get them busy.
Remember- you weild more parental power than you know! One stable parent can do it. I am thinking of you.
Me 41, H 47, M 15 yrs, S11, S13 BD 1: 11/4/14 we work on it; really I pretzel myself BD 2: 3/31/15 H goes down to "dorm room" 8/15: H back to MBR 10/15: H back in dorm room 1/18: H files, now divorced