I suppose it would be preferable, but my 5 year old daughter is still in my bedroom. She suffers from asthma and I find as long as the room is kept well ventilated she is fine. If either parent smoked in the bedroom it would be a completely different matter though.
my daughter has asthma and she shares her room with her sister, apart from somtimes disturbing her in the night coughing i dont see a problem, having anti allergy bedding and not to many teddys helps (put teddys in freezer every so often to kill any mites that aggrivate) and switch from carpet to hard flooring is a must. hoover mattress weekly as well and dust daily, i find its better to damp dust rather than polish as this can aggrivate her to.