Eighty-four demonstrators, including at least two children under 10, were arrested in a protest against the long-delayed Seabrook, N. H., nuclear power plant. Those arrested had climbed a fence with handmade ladders or had blocked plant gates during the finale of a two-day protest against possible low-power testing of the $5.6-billion plant later this year. More than 500 protested over the weekend. Police said nine juveniles arrested were not charged. They said the other 75 were charged with criminal trespassing, a violation comparable to a speeding ticket, and released. Seabrook was completed in 1986, but it has been unable to get a federal operating license. Federal regulators have yet to approve evacuation plans for surrounding communities.