I had my 3 door ed30 for almost 5 years and had ideas of keeping it long into fatherhood... A combination of a narrow drive, my dodgy back and a car seat designed by Satan meant it lasted a week until the 5 door arrived
Having kids doesn't mean you HAVE to get a 5 door, far from it imo, it's entirely down to personal circumstances and even what type of seat and even how big you are, my friends love their 3 door gti as they just get in behind the drivers seat, stoop a little and strap little one in...however they are 5'8" and 5'2" whereas I'm 6' and have had far too many pies over the years and the missus is 6'2", so a 5 door works better for us...
See how you go with your current car, it could well be perfectly fine, if not you can swap it later on rather than a knee jerk reaction to kids must=5 door