Your math is bad (gotta love the likes on this one). It should be:
90K minus 12,700 standard deduction minus 16,200 personal exemption = $61,100 AGI. Tax is 10% of the first 18,650, and then 15% of $42,450, for a total of $8,232.50. The child *credit* (1000/kid) brings tax down to $6,232.50.

Under the new plan:
90K minus 24,000 standard deduction = $66,000 AGI. Tax of 12% = 7,920 minus child *credit* (1600/kid) = $4,720.

Still a savings of $1,512.50, but less than half of what your math shows.