Here is a possible solution using a radio button field
Step 1: Create a new text field, lets call this field "type" for the purposes of this post
Step 2: Place the new "type" field on your layout, set it to be a radio button field and use the Adult / Child value list.
Step 3: Enter browse mode and make sure you are showing all records (Click Show all records in the status bar or in the Records menu)
Step 4: Click into the "type" field and go to the Records menu and select Replace Field Contents
Step 5: Click the "Specify.." button beside Replace with Calculated Result
Step 6: Enter the following Calculation (oldfield is your field with a or c in it)
Case ( oldfield = "a" ; "Adult" ; oldfield = "b" ; "Child" )
Step 7: Change oldfield to a calculation field with the calculation
Case( type = "Adult" ; "a" ; type = "Child" ; "c" )
Since I'd be using 'type' as the active field and 'oldfield' has become merely a data source, what is the rationale for step 7?
I should have mentioned that occasionally I will be updating the contents of this table by importing from Excel, so, one solution would be to make 'type' a calculation field, using the calculation you described.
The reason for step 7 is because if you change the value of 'type' it will also change 'oldfield' if you make it a calc. If you do it the other way around and make 'type' the calculation then you will not be able to use 'type' as a clickable radio button field in browse mode because it will be a calculation field instead of a text field, thus negating it's entire purpose.
If you're importing data from excel, and the data is in the format of 'a' or 'c' then change your field mapping to import that into the new 'type' field (you can't import into calculation fields which your 'oldfield' is now set to). Then in your import script, right after the Import Records script step, add a Replace Field Contents for the 'type' field with the calcultion Case ( type = "a" ; "Adult" ; type = "c" ; "Child" )