While some media reports claim that the Earl of St Andrews' child(ren) "converted" to Catholicism at age 14 or thereabouts, other media reports state that they underwent the Catholic ceremony of confirmation at that age (implying they were already Catholic). For example, a Times article from 2003 stated:
The young lord, grandson of the current Duke and Duchess of Kent and 22nd in line to the throne, will be confirmed as a Roman Catholic in a ceremony at Eton College Chapel. The teenager has been worshipping with fellow Roman Catholic pupils since he went to the school last year.
https://www.thetimes.co.uk/article/people-by-andrew-pierce-lw6x8vrw52c
I believe 14 is a normal age for Catholic confirmation, judging from other Catholic royals. I find it more likely that the confirmation reports are correct, and that the Earl and (Catholic) Countess of St Andrews simply raised their children as Catholics from birth with the confirmation ceremony at the usual age. The scenario of the children being raised Protestant and all happening to "convert" to Catholicism at the same age that Catholics are confirmed seems less believable.
The royal website has also listed Lord Nicholas Windsor's children in the line of succession in the past despite their being raised Catholic, so its accuracy is questionable.