With a child between Mary and Philip, there would have been no reason for the crown to pass onto Elizabeth. The only need would have been if the child died without an heir.
There are very high chances that Mary and Philip would have only had one child together. That would have meant no siblings for the crown to pass onto after the child’s death. Therefore, it would have gone to the next person in line. Mary never officially removed her half-sister from the line of succession, and even with a child I don’t think she would have. She would have put all hope in that child going on to live and have children of his or her own.
However, had the child died without an heir, it is possible that Elizabeth would have become queen. It’s now a big “if.” You see, the child would have been raised as a Catholic and would have wanted to avoid the country moving back into a Protestant country. Elizabeth would have threatened that, so the child may have changed the line of succession—or the regent for the child (likely Philip II of Spain).
There are a few options for a predecessor if Elizabeth was out of the running. Mary, Queen of Scots, remained a Catholic despite her country turning Protestant. While she abdicated from her Scottish throne, there is a chance she would have found safety with Mary’s child as a ruling monarch and that may have led to her becoming next in line for the English throne. Other options would have come from Spain.