Hello Irina, Sorry about this illness in both kids. Belladonna is not for fever but tummy ache. You need to give paracetamol alone for high spikes of fever above 101 *f. It can be repeated every 4-5 hours if needed. Also just give some steam inhalation and warm sips of sugar water with dash of salt.
Majority of drugs for mucus are not effective. You may put some decongestant nasal drops in nostrils at bedtime if needed for easy breathing.
Also raise head end of cots. If there is wet cough, please keep them inclined on your knees head low, and thump over upper back (on their empty stomach) few minutes every couple of hours to remove secretion from upper air-way.
Continued fever over 3-4 days may need consulting your GP for possible antibiotic course.
Good luck to both.