If you wish to show only one child form at a time then use a simple token logic to implement the rule.
Have a global static variable, preferably a string that can store the current child form name, that would act as the token.
On all the child forms' constructor implement the following pesudo code -
if GlobalToken = "" then
GlobalToken = CurrentForm.Name
else
Show Message
end if
On all the child forms' destructor clear the token -
GlobalToken = ""