Easy one day drive but I'd love to extend vacation and find a resort/hotel with a lazy river between 1/3 to 2/3 of the way to stop at for a night.

I googled but didn't find anything and I'm pretty sure no place exists without going far out of the way. Any suggestions? Next best thing? Family of 4 with twin 15 year old girls.

I don't think anything like that exists along that route, at least that early into the trip. Carlsbad Caverns is a reasonable and interesting detour from your route, if you haven't seen it, but it doesn't fit your description.

Be aware there are heavy rains in the Hill County and some flooding is occurring. Will probably have receded by the time you get there, but you might want to check it out.