yes, it does help, thanks. if you were building the database, my first
recommendation would be that you restructure the tables to include a table
listing each child (person, not table <g>), and use one or more child tables
of *that* table to record the various activities associated with a child
(person). but, given your situation, we'll forego addressing the root cause
of the problem, and see if we can get around it at the form level.

you can link the second child subform to the first, within the main form.
i've never tried exactly the following setup, but it might work for you:
make sure both subforms are linked to the parent form with the standard
primary key/foreign key field references in each subform control's
LinkChildFields and LinkMasterFields properties. then add an unbound textbox
control to the *main* form, set its' Visible property to False, set its'
ControlSource property to the name of the date of birth field (i'll call it
DOBa) in the first subform control (i'll call it ChildA), as
=[ChildA].[Form]![DOBa]
you'll need to substitute the correct names in your database, of course.
name the textbox control txtDOB.
in the second subform control, *add* the name of the date of birth field to
the LinkChildFields property. so that property will show two fieldnames:
the foreign key field that links the table to the parent table, and the date
of birth field.
still in the second subform control, *add* the name txtDOB to the
LinkMasterFields property. so that property will show two names:  the
primary key field of the parent table, and the name of the unbound textbox
control on the parent form.

so (hopefully) when you choose a record on the parent form, the first
subform should display all the child records related to that parent record.
and the second subform should display the records related to that parent
record *where the date of birth matches the current record in the first
subform*.

hth