Others have recommended that seat-adapter, Hilda. I'll see if I can find one here (in Germany).
Thanks everyone else for all the tips, too. I am now a fully-fledged Granny. The baby, little Emil, was born four weeks early. I don't know if DIL's friend will still be organising the surprise baby shower, but at least one of your problems with it is now solved - if it does take place, it will be after the baby's born!
Anyway, if this party does take place, it will be great, as the whole clique are fantastic at organising these things and have lots of ideas for games and activities and will no doubt have prepared songs and sketches for everyone's amusement.
My sister-in-law had no idea what it was and thought it was like a Tupperware party. I said "Well, sort of, except you don't have to buy anything and you have to bring a present." She had imagined that the presents were already prepared, like a wedding list, and you just chose which one you wanted to "sponsor" (i.e. pay for). Actually, that is quite a good idea. Hmmm..... I wonder...