I did both the WW 1 & 2 War grave sites cycle camping with my 10 - 12 yr old grandson. Both are great experiences and are doable. We caught a ferry over and back twice and once flew to La Rochelle and cycled back via the WW2 sites. My wife dropped us at ferry ports and picked us up as we are only an hour or so away, no doubt you could get a train. You may well be struggling even to find camping space a Ypres next year so be careful, we went there in 2014 and it was heaving. Cycling around Ypres is really easy with lots of sites but I did find the graves did just get overwhelming for him. For one day we went to Brussels by train to give grandson a break. The Normandy beaches would be my recommendation, easy cycling, lots of campsites, lots of sites to see (USA graves and museum, Omaha beach, Sword beach, commonwealth graves, Peguses bridge etc) just a bit more variety than Ypres. Both are really good and the logistics might be the deciding factor.