Best time for family to travel to Europe?

My family and I are trying to plan a trip to Europe in about a year. It would be 2 adults and 2 kids ages 6 and 8. We are thinking of doing a 6 wk stay and try to hit Spain, France, Londen, Germeny, Italy, and Greece We are trying to find out when would be the best time to visit. We are trying to do this all on a budget.
We are from hawaii.

I would say Spring or Fall. Winters can be very cold over there. Summers are OK for weather, but in August, some countries pretty much go on vacation. I know that in France, not much goes on in August because everyone is on vacation. This means that vacation spots will be very crowded, and regular businesses are usually closed.

croatia is the best. i was there. such a nice sea. where are you from

I advice you to travel late spring or early summer. That way you will avoid too cold or too warm a weather, although the weather worldwide is not what is has used to be. Global warming, I´m sure.

Have a nice visit whatever you choose.