Wat antihistamine help with block nose for toddler due to fluu symptoms i think it more allergy than cold?

I don't recommend them. Antihistamines often thicken secretions/mucus to the point young kids have more problems not less. A simple decongestant like Sudafed may help some. For most, simply keeping the well hydrated will help enough until it passes.

Unless the antihistamine is prescribed to treat allergic nasal conditions, please use a nasal saline spray and gentle suction. This is often effective in reducing the symptoms and post nasal drip. Testing for Flu,RSV and COVID-19 may be helpful. Monitor temperature and breathing. Watch for signs of shortness of breath and respiratory difficulties.