I've only been here a year and don't have little kids. Some of the developments have pretty nice community parks. I live in Creekside Estates (McCreary and Park sort of) and we have a lake with geese, a play area, fountain and some plantings.
I searched Google and found this:
www.ci.wylie.tx.us/Parks/olde.htm
I think there is an old caboose that might offer some cute opportunities. If it's not at the park, it's right on Hwy 78 on the west side, north of FM 544. There are other parks listed at the website.