I am going to make all mumsnetters fume but my daughter was picked up a few times in my husbands van just on her booster seat when she was about 2-3ish. Also if i have had to get a taxi they have sometimes just been strapped, not through choice though.
I cant see why she cant buy one of those cheap booster car seats from mothercare, they are only about 27 quid and are 'safe' from 9 months, then her 4 yr old could have the booster. i take it your son would have a booster too? it is not that much hassle is it?
makes it really awkward for you to say anything though. Would me anyhow