Keep the E90 M3. I had one and it works fine. I have two little kids and now have a rear facing seat and a front facing Recaro in an M5. It's little tight only because I am tall but very doable. Wife has an SUV but honestly, we take my car on the weekend because I like to drive and we all fit fine.
I got rid of my E90 and got...an Escalade!!!....when I had my firstborn and traded it in less than a year later on the M5. Nice and comfy but totally not necessary. Plus I missed my Ms. Thought I needed all that room and size but it is a myth.
2009 E60 M5
2008 E90 M3 - Gone