The refund is ONLY determined by subtracting what you from what you paid in during the year.
If you look at what you actually OWED for 2017, you will see that you OWED much less for 2018, through a combination of tax rates being cut, the child tax credit and an increase in your standard deduction by $11,300 (which more than makes up for the loss of the two dependent exemptions you took for yourselves in 2017, totaling $8,100). I GUARANTEE that if you had about the same income both years, the tax you owe for 2018 is a LOT less than it was in 2017.
So, if you are getting a smaller refund in 2018, it means that you paid in a lot less each paycheck than you did in 2017. That's because the withholding RATES were cut to coincide with the tax cuts. You got MORE of YOUR money every paycheck all year and the government took LESS of it to give you back in a refund.
Which is exactly how you should want it. You should want ALL of the money you get to keep after taxes AS YOU EARN IT, and not wait until the following year to get it back without interest.
Me, I try to set things up so that I owe a small amount every April. It means I got to keep some of the government's money for a whole year.