Hi,
I've been a Master Gardner for 30yrs and am new to Texas as well as the south in general. I have a couple of suggestions, just about any Herb will grow well and tolerate the late afternoon/evening sun as the days get longer and they tolerate in fact don't mind drying out, if you forget to water them for a couple of days. W/ the 4yr old in mind, try to find some bulbs and or sweet potatoe and plant them in clear glass jar or vase, that way your little one will be able to watch the roots grow (they have larger roots to see)and get to enjoy the flower. You could also check out your county Extension Service website or call them. They helped me when I got here and saved me from making some big mistakes.