Please help with reserving tickets for children at Mont Saint Michel

hello, my husband purchased two tickets for Mont Saint Michel for the two of us, and I understand children are free, but there was no option to reserve free tickets for children. It actually said on one site that reserving tickets for under 18 is not permitted. but in the email we received it says that children are free, but must still reserve a ticket online in advance to avoid having to wait in the longer line. It gave us a website where we should be able to do this, and says there should be a “child/minor” option, but there is not. has anyone booked these tickets for children before that could share with me where I can find this option?? Thank you so much!