First of all, what version of Access are you using? All versions of Access are capable of doing what you are trying to do, but the user interface of of Access may be different depending on the version you are using and I want to give you the proper instructions.
When working in Access, you are using two programming languages. One is called SQL and the other is VBA. SQL is the language that is used to retrieve, store and delete data from the database; VBA is the language MS Access uses to change to properties of the forms and objects on the forms. Both SQL and VBA can do more than what I mentioned above, but those are the basic differences. You are able make VBA run statements in SQL; when you do that, VBA basically passes the SQL statement along to SQL and SQL performs the action.
Here is an overview of what you want to do using MS Access 2002:
1) Create a new form (or edit an existing form) in design view.
2) Add the form header and footer. Right click on the part of the form that says Detail and select Form Header/Footer
3) Click on the Combo box icon on the tool bar and create a combo box in the header.
4) A wizard should pop up wanting to help you create the combo box. Click Cancel; we will set everything up manually.
5) Right click on the combo box and select properties.
6) The properties should appear for the combo box. On the data tab, click on the box next to Row Source.
7) A button with three dots should appear next to the box you clicked. Click on the three dots.
8) The SQL builder should now appear along with a list of all your tables. Double click on your children table.
9) Click close to close the list of tables. You should now see the children table in the SQL builder.
10) Drag the ID from the Children table to the bottom portion of the SQL builder. In one of the columns at the bottom, the field should show ID and the Table should show the name of the children table
11) Close the SQL builder
12) The Row Source property of the combo box should now be updated.
13) Click on the Event tab on the combo box properties.
14) Click on the event On Change
15) Click on the three dots again and choose Code Builder. The VBA screen should now appear.
16) Between the lines Private Sub combobox1() and End Sub, paste the code I gave you earlier. Please note that I am assuming combobox1 is the name of your combo box; if your combo box has a different name, then update combobox1 to the name of your combo box.
17) Lastly, make sure the Record Source of the form is the children table. To do that, view the form in design view, click on the box that is above the ruler on the left and to the left of the ruler on the top.
18) The properties box should now show the properties for the form.
19) Select Record Source from the Data tab of the form properties.
20) Click on the three dots.
21) Again, drag the children table from the list to the SQL builder.
22) Drag the * from the table to the columns at the bottom.
23) Close the SQL builder
24) Save the form and you should be done. Please note that you will need to add the fields to the form so they show up.
I believe those instructions should work; let me know if you run into any issues.