hmmm...Hamax website says 'up to 22kgs' (48 pounds) (which is about what my - tall - 7 year old weighs!) Small brother only weighs 30 pounds.
Possibly MrSRD worrying too much, but I gather problem is that Hamax seat is pressing down on blackburn rack, which is lacking a bolt on one side, so then rubbing on mudguards. (I hadn't totally understood the prob when originally posting, since I hadn't seen bike).
So, if bolt fixed then hamax would just bump top of rack. Or you could take off rack (don't really want to do that). I think Mr SRD's feeling is that if Hamax is that bendy, do we really want him in it? even if it is supposedly still under limit?
Also, our Hamax is older than one on website. don't know if weight limits the same or not?