I tend to think that the Westminster Divines overstepped a little when they differentiate between elect children and non-elect children. I believe in something more akin to the old "age of accountability" based on David's response to losing his son (2 Sam. 12), the Jewish custom of going up to the temple at 12 (Luke 2:42), Jesus's love for children (Mark 10) and others. I found John MacArthur's treatment of the topic in his book Safe in the Arms of God persuasive.
Unfortunately, this doctrine isn't something we can put 100% certainty in based on the scripture. It's possible that there is a distinction between elect and nonelect children or that all persons, even children, who die before accepting Christ aren't saved. But based on the weight of the evidence, I lean on the age of accountability theory.
Sure:
Deuteronomy 1:39 NASB Moreover, your little ones who, you said, would become plunder, and your sons, who this day have no knowledge of good and evil, shall enter there, and I will give it to them and they shall take possession of it.
Isaiah 7:15-16 NASB He will eat curds and honey at the time He knows enough to refuse evil and choose good. [16] For before the boy knows enough to refuse evil and choose good, the land whose two kings you dread will be abandoned.
Since your reformed, John MacArthur wrote a book on the topic of little ones passing away and why their in Heaven. I believe the age of accountability was one of his reasons.
Here is an article where an interviewer summarizes if interview with MacArthur about the book. The first question is about the Age of Accountability.
Here are a few things that people gave me that meant a lot:
After my daughter's first birthday has come and gone, I think my fear has been that people will forget her following birthdays, as though a year is enough time and things are done. So cards or messages in the years down the road will mean a lot.