I get where you are coming from with respect to buying a bigger property/further out, however, it's really down to them and your opinions valid, but really you should not get over-invested in this one.
We are in the process of buying a 2 bed ground floor maisonette (with garden) in zone 5 SE London...plus we have 2 DC under 5.
We totally would have gone for a 3 bed house- if we could have afforded it!
Unfortunately, houses round here are totally out of our price range and we do not wish to move further out/down to Kent and then do the whole big commute etc.
My DSis and DM have both waded into things and given their (unsolicited) opinions and it is so very irritating.
DSis has a very well off husband so has no real concept of saving for a deposit, applying for a mortgage etc. I am sick to death of listening to her go at me, saying how we should 'bite the bullet' and go for a bigger property.
She doesn't seem to get the point that we cannot afford it, not that we simply do not want to.
I would respectfully suggest that you share your opinion once, but do not go about it as that will only grate on them during what should be, an exciting time.