If you opt for the Newhaven Dieppe crossing (4hours) IIRC , your daughter will get plenty of time on a large(ish) ferry.
Upon arrival, you will find that Dieppe is an established Norman port town, it has a lovely chateau, various places to eat in the town centre, lots of interesting shops (if that is your thing) A fresh fish market by the Harbour. There used to be a Citroen 2CV factory there at one time. There is also some military interest, Dieppe was the location of the August 19th 1942 raid by the Canadians, it was a precursor to the D day landings,and was intended as a 'dry run' but as far as the Canadians were concerned an unmitigated disaster, but lessons were learned that meant the D day landings went more to plan.
Plenty of evidence of Hitlers 'Fortress Europe' on the cliffs above the beach where the Chateau sits, and from where the Germans were able to inflict total carnage on the Canadians. There is a bench overlooking the beach behind the Chateau, where nearby is a steel observation turret, which looks just like the top of a Dalek!
Along the beach promenade are grassed areas and childrens play parks, back from the beach and into the town there is (or was) I haven't been to Dieppe for a while, a brasserie called 'La Tribune' which serves food and drinks, enjoy a cafe au lait, a beer or glass of wine.
As a family we used to holiday every year at Dieppe, in those days the crossing was made in a SNCF (French National Railways) ferry, the two vessels that crossed from Newhaven were the Valency and the Villandry,(named after local chateaux) both long since rendered into razor blades! the ferries not the Chateaux!
We camped on a site at the top of the town in what had been a WW2 German Radar/Anti aircraft battery barracks.(Great fun investigating the old bunkers) the toilet/wash rooms had been the same ones as used by the occupying Nazi's.
At the back of the site was a velodrome (of sorts) which was basically a flat cycle race track, in August there used to be local races held, which were always well attended.