IRS documentation says you can only do this for "An absence of 2 weeks or less is a short, temporary ab- sence. An absence of more than 2 weeks may be consid- ered a short, temporary absence, depending on the cir- cumstances." I am not sure if this is an extenuating circumstance, since (1) we are not on leave to care for the older children, and (2) we cannot pull them out of childcare and automatically have them allowed back after this leave, which would be necessary for us to work. If relevant, this would be for a 2-mo leave for the parent with the FSA and a 2-mo leave for the parent without the FSA.
