I'm sorry , I have no experience with singulair and am not aware of the severity of you DD's asthma, but would it not be worth just giving the switch from flixotide to seretide a trial before adding another drug? I was on flixotide for a while but found I was still needing ventolin at least once a day. It really felt not different to when I was not even on a preventer. This year I started taking seretide and do not need my ventolin at all, with the exception of being sick or strenuous exercise
etc.
Is the singulair something to take regularly as a preventer, or is it for when she has an asthma attack?
I am in no way comparing my situation with your DD's, it seems her case is much more severe than mine and of course I can't really compare an adult with a child, but I just thought I'd share that with you anyway
Good luck with it all, I hope your DD doesn't suffer any of the negative side effects you listed above.