The lift's capacity is equivalent to 18 adults or 30 children, meaning:

1 adult = $30 \div 18 = \frac{5}{3}$ children.

If 12 adults are already in the lift, their capacity in terms of children is: $12 \times (\frac{5}{3}) = 20$ children.

Remaining capacity for children = $30 - 20 = 10$ children.