I've been trying to figure out the name of this movie I saw in the mid 70's about these kids who lived in some old house and they ended up killing people who were staying there. One scene showed a kid on a swing with a machete attached to it swinging down and impaling a guy. Another scene showed a kid pouring a bucket of piranhas into a bathtub that a woman is taking a bath in (she had a washcloth covering her face I believe). It freaked me out back then and I'd really like to try to find it again

Devil Times Five (1974) ?

Could it be Devil Times Five?

The kids did not live in the house originally. The children were patients of a mental institution (for children with severe problems) and they were traveling in a van. There was an accident, the driver was killed (or they killed him, cannot remember that exactly). The children got away and found this retreat house. The people staying there were there for a vacation of sorts. The children killed the caretaker and invaded the house.

Yes, they did kill everyone. The piranha fish in the tub with one of the women, the impalement, etc. After it was all done, some of the children made "dolls" out of the people and had a tea party. I remember one of the girls stroking the hair of one of the dead women.