my daughter takes singulair and zyrtec simultaneously. she does not see ANY side effects from them. not a single one. She actually gets 'dried out' from benadryl... she gets bloody noses and gets dry eyes when she takes it.
all you can really do is try it... if she has one of the negative side effects, then try something else.
every medication, natural or synthetic has a whole list of side effects... you will never find one without it...the key word is 'possible' side effect. they put those on there to differentiate between a known side effect, and an allergic reaction.
since singulair went over the counter though, my insurance company has dropped it from their formulary, so check to see if it's even covered.
good luck