It's very simple. Just use the cut and paste functionality of your editor of choice to move the 'child' template out of the 'parent' UDS and into the main body of your script. Just remember to put it someplace before the 'parent' dialog's Dialog function gets called.
Below are my Child dialog lines. The Child UDS is defined near the top of my script, above the Parent UDS. If I take the "Dialog lines" below out of the Parent UDS, and place them BEFORE the Parent UDS, the script displays the Child dialog first, rather than the Parent dialog.
SelectFormat=`WWWDLGED,6.1`
SelectCaption=`Select Item`
SelectX=9999
SelectY=9999
SelectWidth=186
SelectHeight=251
SelectNumControls=012
SelectProcedure=`SelectCallbackProc`
SelectFont=`DEFAULT`
SelectTextColor=`DEFAULT`
SelectBackground=`DEFAULT,231|230|222`
SelectConfig=0
Select001=`143,233,036,012,PUSHBUTTON,DEFAULT,"&OK",1,5,32,DEFAULT,DEFAULT,"231|230|222"`
Select002=`101,233,036,012,PUSHBUTTON,DEFAULT,"&Cancel",2,6,DEFAULT,DEFAULT,DEFAULT,"231|230|222"`
Select003=`003,005,170,008,STATICTEXT,DEFAULT,"Enter contact, or select a contact from the Address Book below.",DEFAULT,3,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select004=`003,047,134,010,EDITBOX,NewAddress,DEFAULT,DEFAULT,3,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select005=`003,027,066,010,EDITBOX,FirstName,DEFAULT,DEFAULT,1,@csCurLeft,DEFAULT,DEFAULT,DEFAULT`
Select006=`005,019,030,008,STATICTEXT,DEFAULT,"First Name",DEFAULT,3,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select007=`073,019,030,008,STATICTEXT,DEFAULT,"Last Name",DEFAULT,3,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select008=`071,027,066,010,EDITBOX,LastName,DEFAULT,DEFAULT,2,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select009=`005,039,046,008,STATICTEXT,DEFAULT,"Address",DEFAULT,3,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select010=`003,071,174,156,ITEMBOX,AddressBook,DEFAULT,DEFAULT,4,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select011=`004,063,170,008,STATICTEXT,DEFAULT,"(Double-click an entry to remove it from the Address Book)",DEFAULT,3,DEFAULT,DEFAULT,DEFAULT,DEFAULT`
Select012=`003,231,070,016,VARYTEXT,localdir,"localdir for callback procedure function only",DEFAULT,16,1,DEFAULT,DEFAULT,DEFAULT`
ButtonPushed=Dialog("Select")