We're in East Dunbartonshire and lost four days at the end of last term to strikes. Teachers had to stay away too as the Council said they weren't allowed to be in school without jannies and cleaners. Council handled it spectacularly badly - we weren't told until 2.45pm on the Wednesday that the school would be shut the following day. Hopefully Glasgow will be more on the ball.
So yeah, take the strike seriously. They've walked out in other areas and are obviously prepared to follow through on the threat.
When we were on strike here there were a lot of companies stepping in with options - Goals on Great Western Road was running football camps, the tennis club was offering a day of games and coaching for £20 and so on. And a lot of "you have mine thursday friday and i'll have yours next week" type arrangements. I'd start planning for the worst, hoping for the best.