A base for forest hiking with younger children?

I would like to plan a family holiday in the next two years. We currently have an 8 and 5 year old, plus a baby on the way.

The two who are already born are regular hikers, but the younger one maxes out at about 7km.

Is there anywhere in Europe that meets the following criteria:

• Lots of day hikes suitable for children of this age (or a bit older, if we don't go till 2027).

• Accommodation (can be camping or brick and mortar, but if camping we'd want to be able to hire everything there).

• Based in an area with lots of forest cover, ideally including older growth and high biodiversity, rather than plantation monocultures.

• Good hiking weather either over Easter, or in August. We are based in the UK and Scotland, for example, is a no go in August because of midges; Mallorca likewise because of heat.

• Ideally water around (lakes, rivers, etc.). My wife loves hiking near water.

Bonus points if all or most of our time could be spent in a national park or similar protected area, where there is reasonable access to amenities, but lots of the trails are in areas where construction is highly limited or forbidden.