On rainy days we usually end up having friends with kids over or head to someone elses (or abuelas). Burns a few hours of the afternoon.
But here's a list of options:
You can take them to the play centres at Unicenter or Dot, expensive but if you're desperate, anything's worth it, haha.
I haven't taken mine yet to prohibido no tocar, not sure what age it is for, also tomorrow a holiday so probably closed?
Kids also tend to love walking in the rain, i've got boots for mine so even that burns a bit of time. Is yours still napping?
You can even just take your kid to a regular cafe with music (we've taken ours to a place that has live jazz off of Plaza Saavedra, he has a bit of a dance and a media luna and is a happy boy). Or go have mate / coffee at Las Cholas and they can draw with crayons for a bit.
You don't have to limit yourself to the places with peloteros. If you're willing to pay the money you can go to places like Marcelo -- expensive but also pretty family friendly, there's always tonnes of them about. We also do Tucson / Kansas with the tablet in tow. (There's a Tucson at DOT, expensive night but burn them out on the rides at the play centre, rent them a little push cart car to "drive" around the mall in, and finish with ribs etc at Tucson downstairs.).
http://www.revistaplanetario.com.ar/ (has a whole bunch of paseos / teatro / actividades)