I think it simply wasnt meant to be of significance when Ryouko held the baby at its clothes. If it was and she´s found out because of it, Id agree shes being careless and stupid. But as of now, Ill assume that it was meant more for us, the audience, to get another glimpse at her ruthless behavior towards her baby, rather than to introduce a future flaw in her disguise.

My two hopes for the future:
1.) The detective not being killed any time soon. I want him to be more fleshed out. Let him be Runge-san.
2.) Shinichi making a push to help ruining the opposing parasytes. No more hiding and doubting.