Question 65. Since then we are made partakers of Christ and all his benefits by faith only, whence does this faith proceed?
Answer: From the Holy Ghost, who works faith in our hearts by the preaching of the gospel, and confirms it by the use of the sacraments.
So reads Q&A #65 of the Heidelberg Catechism.
Zacharias Ursinus, the primary author of the catechism, has some interesting comments on this answer:
Faith is begun and confirmed by the word; by the sacraments it is only confirmed, as in the supper. The word teaches and confirms without the sacraments, but the sacaments not without the word. Adults are not saved without a knowledge of the word; but men may be regenerated and saved without the use of the sacraments, if this omission be not accompanied with any contempt. The word is preached to unbelievers and wicked men; the church should admit none to the sacraments, but such as will have us to regard as members of his kingdom (p. 356; emphasis added)
Notice, that Ursinus believes he is speaking particularly about adults converted to Christianity. If, on the other hand, you want to know what the Reformed believed about infants, you have to read what they say about infants.