Indeed you do understand what the Canons are teaching in Article 17 correctly. This is a classic statement of the infamous "Presumptive Regeneration". As I have so often stated here and elsewhere... I categorically reject this one article of the Canons of Dordt to which I have a great respect and love. I find not one shred of evidence in Scripture to support that view. And interesting enough, the authors of the Canons don't supply even one single passage to support this article. It is more often than not that 1Samuel 12:14-31 is referenced, and particularly verse 23 to give proof that David's son Chilead, aka: Daniel whose life was taken as an infant by God for David's sin of being guilty in the death of Uriah, was saved. But if one reads this verse carefully and without a presuppositional bias, David is simply saying that he (David) would eventually die and be as his dead son. And if one wishes the text to mean that David would actually see his son after his death, there is absolutely no warrant to make this a universal doctrine of salvation of infants of believing parents who die in infancy are infallibly saved. The Westminster Confession, Chapter 10 - Article 3 I believe states the truth in this matter as does the Savoy Declaration:
III. Elect infants, dying in infancy, are regenerated, and saved by Christ, through the Spirit, who works when, and where, and how He pleases: so also are all other elect persons who are incapable of being outwardly called by the ministry of the Word.
Peter, in his Pentecostal sermon makes it even more clear with these words:
Acts 2:37-39 (ASV) 37 Now when they heard [this,] they were pricked in their heart, and said unto Peter and the rest of the apostles, Brethren, what shall we do? 38 And Peter [said] unto them, Repent ye, and be baptized every one of you in the name of Jesus Christ unto the remission of your sins; and ye shall receive the gift of the Holy Spirit. 39 For to you is the promise, and to your children, and to all that are afar off, [even] as many as the Lord our God shall call unto him.
The bolded part of the text is speaking of the "effectual calling" of God to whom the promise of the Holy Spirit is given, which is ONLY applicable to the elect (cf. Rom 8:29,30).
What can be incontrovertibly presumed
based on the infallible Word of God is that ALL without exception are born dead in sin and under the just wrath of God for they own the punishment of their father Adam, i.e., Original Sin; imputed guilt and corruption of nature. (Eph 2:1-5; Rom 5:12; 1Cor 15:21,22). Being born to one or more believing parents has no inherent guarantee of salvation to the children, whether they die in infancy or grey haired.