You're a bus driver and you don't know???
Perhaps someone with a membership can do a search, but this has been covered before. Short answer:
US84 to Roscoe. I-20 through Abilene. Highway 36 to Temple. Loop around Temple, continue on 36 to Caldwell. Highway 21 toward Bryan, Loop 47 (?) around B-CS, comes in by Easterwood field.
There are variations, such as heading due east out of Lubbock, and picking up highway 6. Some folks will cut south at Sweetwater and take some back roads that run south of highway 36. But for the absolute straightest shot, that's the way to go.
Also the shortest distance is to go through Temple and pick up highway 53 then 485 to intersect highway 6 just north of Hearne. The downside is that you have to drive through Temple and the developing area to the eat of it, as well as several little communities and intersections where you have to slow down and stop. It is also narrow and twisting and sometimes crowded with slow drivers. If the 7-year-old is subject to car sickness, don't go that way. But if you like a challenge and it is still daylight, it can save a few minutes.
Easy gas/food/potty stops: Post, Sweetwater, then most of the little towns down 36, except the REALLY little ones like Gustine, Rising Star, etc. Hamilton is a little tricky because there aren't any food/gas stops on 36, but if you turn north downtown on 281 for a few blocks, there are a few places. Temple has several easy stops, but it seems like Abilene has darn few that are easy to get to.
Long stretches with no easy stops: Post to Snyder and Sweetwater. Sweetwater to Cross Plains, Gatesville to Temple.
You can make the trip in abuout 6.5 hours, though I'd allow 7 to 7.5 with a kid.
Or you could just get rich and fly your own plane down....
