Republican Sens. Marco Rubio and Mike Lee criticized the Senate GOP tax reform plan Thursday evening, saying that its proposed $1,650 child tax credit is not large enough.

"While we are glad to see an increase to the child tax credit, like the House bill, it is simply not enough for working families," the senators, from Florida and Utah, respectively, said in a joint statement.

The Senate Finance Committee briefed Republican lawmakers on the tax legislation Thursday in the hopes of marking it up in committee next week.

The plan calls for raising the child tax credit from $1,000 per child to $1,650.

Rubio, however, has said that $1,800 is the minimum needed to restore the value of the credit to what it was in 2003, correcting for inflation.

He and Lee said it should "preferably" be increased to $2,000 and made refundable against payroll tax liability.

"The Senate is not going to pass a bill that isn’t clearly pro-family, so we look forward to working with our colleagues to get there," they said.

Republicans face difficult math. They are trying to provide large corporate tax breaks in the bill but must limit the total net tax cut to under $1.5 trillion over a decade to pass it through the budget reconciliation procedure that will allow them to avoid the Democratic filibuster.

The House version of the tax reform bill, which advanced out of committee Thursday afternoon, raised the child tax credit to $1,600. The bill also included a $300 credit for parents.

Senate Finance Committee aides said that, rather than including such a parent credit, they aimed to provide tax relief to families through more and lower individual tax rates. The lowest rate in the Senate bill begins at 10 percent, versus 12 percent in the House.