Welcome in H.C.M.
I have gone through your query regarding swallowing problem of your 4 years old daughter, though her upper G.I.endoscopy
As you said that your daughter hold swallowed food at larynx
and white mucous is also there, all these indicates that some inflammatory pathology is going on at her laryngopharynx and this may be ruled out by Direct Laryngoscopy
by your ENT doctor.
Till you are waiting for specialist consultation you may give these to your kid-
1- Steam inhalation.
2- Anti histaminic.
Above will help to your kid.