Trump didn't do anything other than sign a bill passed by the Republican Congress. The bill eliminated a personal exemption and deduction for child dependents, but increased the child tax credit from $1,000 to $2,000. Of the $2,000 credit, up to $1400 is refundable, meaning that even if you owe no tax you might still qualify for a credit.
And, as Volvogirl indicates, the change in the credit is not applicable for the 2017 tax return; this won't start until tax year 2018.