No the child tax credit s not doubled for 2017 tax returns. The increase will be effective for your 2018 return that you file next year in 2019.
https://turbotax.intuit.com/tax-tips/irs-tax-return/2017-tax-reform-legislation-what-you-should-know...
The child tax credit will double next year. See below for other changes that will take place next year also.
https://turbotax.intuit.com/tax-reform/