You can file a joint return with your spouse if you file an election to file a married filing jointly return.
http://www.irs.gov/Individuals/International-Taxpayers/Nonresident-Spouse-Treated-as-a-Resident
You can claim an exemption for your US citizen daughter but you cannot claim an exemption for your son and he was not a US resident in 2018.
Your daughter would be a US citizen, and as she stayed with your wife the whole year, you can enter 12 months.
For your son, you would enter that he is not a US citizen or resident alien and that he lived with you the entire 12 months as he lived with your wife.
