button {
  float: right;
  margin: 0;
}

body {
  margin: 0;
}

.parent {
  width:30%;
  display: inline-block;
  border: 1px solid green;
  margin:20px;
}

input {
  width:calc(100% - 4px);
}

.child {
    padding:50px;
    border:1px solid red;
}