I'll get flamed for this but you could probably acquire an early Octavia vRS 2.0 TDI PD (hatch if you really must!) for £10k.
With an uprated brake pad compound and a breathed on ECU, you have an occasional track car returning 43mpg per tank minimum and 50+ on the m-way. A decent chassis to enjoy on the B-roads. And all the space you will ever need for baby smash.
Other contenders would be SEAT Leon FR TDI, probably better chassis out of the box and whilst it's a slightly smaller boot, it looks less like a family barge / repmobile / Central European blandmobile (delete as appropriate) yet still accomodates all of you in comfort. I think the Megane idea could be a stroke of genius though.
Alternatively, abandon the track day idea - sort of like I did, for a whiley - and get what's right for family and B-roads only. Chances are some trusting and accomodating pals will let you drive their car(s) on track until such times as junior is old enough to enjoy being out paxing in a sportscar at which point you can pick up another Elise
