gochild tsumego

[sobriquet]

Hi.

I'm a bit confused about this tsumego:

http://eidogo.com/#1kcKmpwB4:0,0

It's on gochild in this collection:

Dong-Dong grouping - (1) ~10k_6350 - (6) 20k~10k - (4) - 5789

https://gochild2009.appspot.com/?locale=en_US

In the version at eidogo, I've shown two solutions. Only the first
solution is accepted by gochild. The only possible reason I can think
of why the second solution isn't accepted, is that it has the
potential to create a ko-threat, but it would take two moves, so it's
like an indirect ko-threat.
This seems a rather far-fetched reason to reject a potential solution
to a tsumego, is could this really be a valid reason for accepting
only the first solution?

greetings and thanks in advance for any feedback, Niek