What is the ruling on praying in trousers that have small holes that reveal the skin of the thigh during prostration, knowing that they are sometimes covered by hand? And is it obligatory to make up previous prayers that were performed in these trousers, even if their number is unknown? And is previous ignorance of the ruling excusable?

Concealing the 'awrah (private parts) is a condition for the validity of prayer, by agreement of the jurists. If one intentionally exposes their 'awrah during prayer, their prayer is invalid.

Therefore, it is not permissible to begin prayer with even a small part of the 'awrah exposed. Prayer is not permissible in torn trousers unless they are covered or one is unable to find other clothing.

As for past prayers performed in such trousers, if the exposure was minimal and you were unaware of the ruling, it appears permissible for you to adopt the view of those who do not consider the prayer invalid due to the exposure of a small amount of 'awrah, as is the madhhab of Ahmad and Abu Hanifah.