If I paid childcare from sept 2017-mar 2018 that wasn’t claimed on 2017 return, can I claim it on this year’s return?

No, you cannot claim child care expenses paid in 2017 on your 2018 Tax Return.  However, you can Amend your 2017 Tax Return to include the expenses.  You can click here for instructions on Amending your tax return: How to Change a Tax Return You Already Filed.