Untitled

body {
  margin: 0;
}

div {
  display: flex;
  margin: .5rem;
  padding: 1rem;
  border: 3px solid black;
  min-height: 1rem;
  flex-grow: 1;
}

.grandparent {
  background-color: #c43131;
  flex-direction: column;
}

.parent {
  background-color: #41b443;
}

.child {
  background-color: #7ec9e2
}