true. If he loses then James' credibility would be destroyed. If he's killed would there be an attempt to recall his son the old Pretender and place him on the throne, perhaps with William and Mary as regents. Or maybe the opposite with William and Mary taking the throne and James Francis as heir, especially if William Duke of Gloucester dies as in OTL.
Edit. If recalled he would obviously be raised as a protestant.
The "Old Pretender" was an infant boy. James sent him and his mother to France in secret on the night of 9-10 December 1688. They slipped out of Whitehall Palace, crossed the river in a skiff, and went by coach to Gravesend where a ship was waiting.
James himself fled on the night of 10-11 December, but was caught by some fishermen and returned to London on 16 December. On 17 December, William's troops occupied London, and James was forced to leave the city the next morning. He went to Rochester under escort; and on 22 December fled to France himself. According to Macaulay, William
wanted James to flee the country, and made sure that no one interfered.
It does seem entirely possible that the Queen and Prince might have fallen into William's hands. However, it also seems to me that he wouldn't want to have them. As with James, his position was all the stronger with them gone.
The replacement of James as King was conditioned on his
de facto abdication by fleeing the country. Hardly anyone was willing to say that Parliament could depose the King. William was not going to remove James by force or have him executed; it would have been politically disastrous.
Another point: James was, according to Macaulay, in a near-total funk, confused and demoralized, and in terror for his life. But it still seems unlikely that he would flee the country and leave his wife and son behind.
Perhaps it would work if James chose to fight, but his army mostly deserted, he was killed, and his wife and son captured.
The boy cannot abdicate, and he is the rightful successor, which is very awkward for William. I think William has to settle for a Regency. And yes, young James will be raised as a Protestant.
Or, the Convention Parliament that meets finds some excuse for declaring that James abdicated, both for himself and the boy. Legalisms can be set aside in times of crisis. Then send the boy and his mother into exile. Keeping him around just invites a new War of the Roses.
And William wants to be King, not Regent.