Scores of homeless mothers, who camped with their children overnight in the Massachusetts Statehouse in Boston, pleaded with legislators for increased benefits so their children can be reared in proper homes. Many of the women testified at a hearing before the Ways and Means Committee on a budget proposal to increase welfare benefits 6% in fiscal 1988. "I don't think anybody here wants to stay on welfare," Monica Zani, 19, told the committee. "But we're in a trap and we can't get out of it." Benefits in the Aid to Families With Dependent Children program in Massachusetts average less than $500 a month for a family of three, or 37% below the federally defined poverty level.