e90 m3 here, 4 y/o and 14 month old kid. I've had the e90 m3 since my youngest was a new born. Definitely will have enough room, just need to find the right car seat as well. It's the perfect compromise of kid hauler and weekend spirited driving car...Current set up is my 4 y/o is in a forward facing recaro seat behind me and my son is rear facing in first years car seat behind the passenger seat....once he goes forward facing it'll be a bit more comfortable for my wife to sit in the front seat (she's 5'5", but likes to sit like she's 5'10" lol). But we rarely use my car when all four of us go somewhere, that why she has and when we use her honda pilot
my brother has a 335is, he sometimes takes my oldest out...so I've had experience installing a car seat in an e9x coupe along with getting a kid in and out of the car, I could see how that could get really old really fast. Trying to maneuver an infant carseat in and out of the coupe isn't the easiest thing to do. If the child is older and forward facing it would be easier..but rear facing it's just tough IMO
congrats on the coming new addition to your family...