Not affiliated with the government
Please contact Social Security directly for confirmation, but for the case with two children, and one of those becoming an adult, it seems unlikely the remaining minor will see an increase in benefits.
From http://ssa.gov/pubs/10085.html#howmuch :
Within a family, a child may receive up to one-half of the parent’s full retirement or disability benefit, or 75 percent of the deceased parent’s basic Social Security benefit. However, there is a limit to the amount of money that can be paid to a family. The family maximum payment is determined as part of every Social Security benefit computation and can be from 150 to 180 percent of the parent’s full benefit amount. If the total amount payable to all family members exceeds this limit, each person’s benefit is reduced proportionately (except the parent’s) until the total equals the maximum allowable amount.