It was also much easier to write the tool in Python than in C. Writing it in C would have taken much longer, and at the end of the day we wanted to get work done instead of worrying about a few MBs, which only take 1 second to download and which cost $0.001 of disk space.
In particular you want to avoid memory allocation so it cannot fail even in low memory situations, which you cannot guarantee with Python. That means you might die reaping children with low memory which would kill your container.
Particularly when there are simple init scripts like [1] which could be easily adapted to fill both the reaping and restart problem.
Writing my_init took much more time than you think, and I do not regret choosing Python. My_init not only handles process reaping, but also environment variable handling and bunch of other stuff. Writing text parsers in C is a huge pain (libc has no regexps that are as easy to use as Python).
I do not oppose rewriting it in C, but given limited manpower and the tiny benefits it's not high on my todo list either. If you are worried about resource usage, then I would very much welcome contributions for rewriting it in C.
Or if you want small have a look here for minimal PID 1 "So how should init be done right?" http://ewontfix.com/14/
The environment variable handling and other stuff could perhaps be handled in a separate bash-script (or Python), such that the scope of the actual reaper is sufficiently small (to use a lower-level language).
There comes a point where you have to take a look at the diminishing returns and say "That's good enough."