From experience- include a copy of the divorce decree stating what kids you claim with your taxes, and send it in paper form.
Also, if either one of you uses the social security number of one of the kids you are not claiming as a deduction, (for example, for a child care deduction or college savings account), it will get rejected by e-filing if your ex files first. This doesn't mean you can't claim childcare expenses for a kid that you are not claiming for a deduction, but the computer system will automatically rejects it because the social security number of the kid already appears on another return. You have to send it by paper.
I think you can claim head of household if you were divorced in 2017 but I don't know for sure!
I disagree with this. Don't send a copy of your divorce decree or custody arrangement to the IRS unless they ask for it. E-file as normal.
I also disagree that you'll receive an E-file rejection for the above mentioned issues. You can get a dependent E-file rejection if someone accidentally claimed the dependent first - which is extremely common when people are using TurboTax or other self-preparer software and trying to use the mixed-use dependent credits mentioned by DayLight without actually claiming the exemption.
As for the question about half the household expenses for the children - you would want to consider the entire year. But child support is actually money provided by the recipient of the support (as is governmental support).
Focus on the items that the IRS would ask for if they questioned your HoH status: a lease with your name on it, rent/mortgage payments in your name, grocery/utility/other household receipts, etc. Can you produce proof you paid those things?
Edited to add: HoH can be confusing for divorced parents. But keep in mind the qualifier is: "You paid more than half the cost of keeping up a
home for the year." It doesn't ask where you got the money from. If someone hands you child support, you might use it for the home, or you might use it for school, or you might use it for a trip to Disney. The person paying the child support doesn't decide what you used it for. It got mixed in with your other money, and then you used your money to pay bills. If those bills included more than half the cost of keeping up the home, then that satisfies the HoH qualifier. The custodial parent can choose to release that exemption to the non-custodial parent via Form 8332, but they maintain the HoH status - the non custodial parent doesn't get to claim it.
Some examples of where you might not qualify:
1. Instead of paying you child support, the other parent keeps the mortgage in their name and pays it directly.
2. You live with your parents in their house, and they pay for their home.
3. You live with your boyfriend, and he pays for the home.