"Hello, I have 2 sons and 3 daughters and I'm looking for a job." "OK. And what else can you do?" :-D
Should not it be "mam dwa syna i trzy córki" because it is less than 5?
No. That sounds like a transfer from Russian grammar. Or maybe some other Slavic language that looks similarly to Russian in this matter.
The only time "syna" would be used with numbers is if you had X and a half son. Kinda macabresque.
And BAM! Lesson 1 of numbers and we are already on masculine personal and '5 chairs'.