We have always used passports as the preferred means for identification, even when our kids were small.
Travelling with children - If the parents last names are different (Mom keeps her maiden name), or blended family with the kids having different last names - a long form birth certificate (here in Canada) showing both parents names is a must for showing at the border..... As well, for blended families, a permission letter to travel from the other birth parent (if the case) is good to have as well. Some border agents will ask to ensure the children are freely traveling with the one or the other parent.
Not sure if any of these scenarios apply for your situation - but just in case others read this thread.
Brian