Is Renton considered an ideal family city? No.
Can a family live well in Renton? Yes. Many families do.
Renton struggles with higher rates of violent and property crimes and a higher lower-income population, as does Kent. Both cities are working on revitalizing their images. Renton has a good location and Kent has a great retail/dining/theater development in Kent Station. Kent also offers commuter rail to downtown Seattle.
I have a hard time saying "don't live there" because, obviously, a lot of good people do. But in my experience with relocating to the south King County area with kids (from Snohomish County, to the north), with schools being #1 priority, we decided against Renton and Kent.
We didn't look at Renton. Kent schools have a generally good academic and sports reputation. But the downtown Kent/East Hill area can be rough. Lots of dense, old, poorly maintained apartments and aging schools that appear run-down on the outside. We talked with a local principal who spoke highly of the schools and said they are outfitted with new technology, but overall neighborhood felt rougher than I wanted to raise my family in.
To the east of "downtown", Kent has brand-new schools and the Covington area has quite a good reputation for families. We ruled it out for other reasons, but know a lot of families that highly recommend it.
South King County used to be farming communities that sprung into warehouses and industrial parks and then housing developments sprawled in the fringes. Growth has been kinder to some areas in the region more than others. We ended up in Lakeland Hills, to the South of Auburn. To us, that felt as close to ideal as our budget allowed. Excellent nearby schools, shopping, dining and services within a one-two mile radius, great parks and not to far from major freeways.
As with most things on this site, this is just one opinion based off my own biases and preferences. Your mileage may vary.
Some helpful sites for school information:
*
The Seattle Times School Guide - research area schools, test scores, offerings, etc
*
http://www.greatschools.net/ - school reviews from the general public