All the reasons you banked for the first child are still valid for additional children.
If you want your baby to have the option of using his/her own cells, then you need to bank them.
If you are banking to cover siblings, then the ability to use cord blood from one child for another depends on whether they have matching HLA types. Two full siblings have a 25 per cent chance of being a perfect match, a 50 per cent chance of being a half match, and a 25 per cent chance of not matching at all. For a cord blood transplant, donor and patient must match at least 4 out of 6 HLA types. The more siblings with banked cord blood, the more chance that they cover each other for possible transplants or other therapies for which sibling stem cells are accepted.
References: Odds of sibling match are based on haplotype inheritance: that the child will receive 3 HLA types as a group from each parent.
Source: Parent’s Guide to Cord Blood