I live in Pflugerville with a 2 year old. Honestly, we go into Austin for a lot of our activities.
We do go to the Play for All Abilities Park off AW Grimes in Round Rock a lot.
There's a big story time at the Pflugerville Library and they have some other programs that the 4 year old might like.
I've only been walking at Lake Pflugerville, but a lot of my neighbors like it for swimming/fishing/playing. Beware there is almost no shade and it can get miserable hot.
There's a place off 79 in Round Rock called Safari Champ that's an indoor playground. We haven't been lately since my daughter's in a weird phase where she outgrew the toddler area but isn't confident enough for the big area.
Otherwise we go to Town Lake/Zilker, the Thinkery, Austin Aquarium, and the Zoo (which is a pain in my ass to get to from Pflugerville).