To do it using PID which will just make your life more complicated!!!
In the first script (the important one/the one the others need to wait for) put the following code at the top.
...

ok a very simple way to do it would be at the start of the important script that the others have to wait for add the following line.

touch /tmp/wait

then at the end of the script do the...

you could put a flag in a file in /tmp that the script could check for to make sure the last script completed before starting the next one!