The immediate family of the deceased take priority in planning the funeral (this is the surviving spouse and/or children of the deceased.)
However, it is certainly courteous to try to plan the celebration of life for a day that out of town family can attend. This usually involves choosing a day about a week later to give the family time to make travel arrangements and get there. I’m not sure how “out of town” the family is – be it across the state or across the country – and that will affect the time considerations for planning.
Also keep in mind what the deceased would have wanted – if it was important to them to have this family there for the celebration, then the adult child should absolutely try to accommodate that – within reason.