There are two separate parts to this, each one needs to be setup and working correctly in order for the emails to be sent. The first part of this is making sure that the final list of values makes use of the email address as the value of that list. When I checked on your Dynamic Dropdowns server behaviors on the most recent page you sent I can see only one create dynamic array. I also do not see any 'populate list from array' behaviors applied to your child select list.
In order to populate the child list successfully you will need to ensure that you have the dynamic dropdowns setup and working properly. It looks to me like you need another create dynamic array from the email rs. When you set this up you will need to relate it to the child list via the foreign key that they share. Once you have this dynamic array created for the email recordset you can then apply the 'populate list from array' behavior to the onchange event for the child select list. Make sure that you are populating the email list with the email addresses as the values. It is ok if these are also the labels.
Once you have this third select list populating with the email addresses as the values you can then use this select list for the 'to' setting in your UE server behavior. You should get it working as a single select list first, then update it to a multi select list afterwards.
