How is it possible that 3 yrs daughter & 4 yrs son had exactly same symptoms i.e. strawbery tounge fever & sore throat but swab test after culture for strep throat is only +ve for daughter.

Both have strep : Strep tests can be falsely positive sometimes, would treat both as a strep infection with antibiotics

False negative : Or false positive tests can be the case, although, uncommon, the test can repeated for the 4 -year old, however given the symptoms and the clinical picture, I would treat both siblings for strep throat, wish you all good health