As a general rule, each parent is responsible for their own travel expenses (such as for holidays) when the child is in their care.
In circumstances where a child is required to travel a significant distance to spend time with the other parent, the general “rule of thumb” is that this cost be borne by the parent who will be spending that time with the child.
However, if the primary caregiver has relocated a significant distance away with the child post-separation, it may be appropriate for any travel expenses (e.g., plane fares) to be shared equally.