flex-shrink 足りない幅を子から減らす

<div class="row">
  <span class="keep">縮まない</span>
  <span>こちらが先に縮む</span>
</div>

.row {
  display: flex;
  width: 12rem;
  gap: 0.4rem;
}
.row > span {
  background: #efeef8;
  padding: 0.4rem 0.5rem;
}
.keep {
  flex-shrink: 0;
}