Thank you all !
My twin baby boys (1yr old) started taking Singulair on Friday and we've all had a hard time since then: they don't sleep well, cry a lot (the "I'm in pain" kind of crying), etc ...
I suspected the drug to be at the origin of all this but was not sure. Now that I've read all these posts, that's it for us: no more Singulair; their allergy symptoms do not justify such side effects !