What a nice idea! It is so rare to see generosity anymore. From my experience as a child, I would recommend that you don't create a solid woodland but pockets of trees surrounding large open areas. This way the children could play hide-and-seek among the densely wooded pockets but still have room to run around and start a kickball game or fly a kite.
One word of caution. If you live in the United States, I would be concerned about the eventual lawsuits filed by parents, who should have been monitoring their own kids but, instead, they sue you for the injuries that all children get when playing because the injuries occurred on your property. I'm sure others can chime in about some horror stories. This is why generosity is so rare. It is a sad commentary on what our society has become. But perhaps there is some sort of reasonably priced insurance you could purchase to cover this eventuality.
The spacing of the trees is entirely dependant on their mature size. Look at the native trees in your area to determine natural spacing and try to emulate this. Then choose which areas will be forested and which will be open. Then you can make a guess as to how many trees you will need. And don't forget the understory plants--they make the best hiding places! Good luck with your project.
In the temperate climates of the USA the general advice with conifers (pine, fir, spruce, larch) is to plant them 8' apart then plan on thinning them to 16' apart when they fill in, usually after 20 years. I did this with 3 acres and it has worked beautifully. They are not 40 years old. I used eastern white pine, norway spruce and european larch. You wouldn't want spruce where children play since it has a stiff sharp needle. Also, larch is a deciduous conifer but is absolutely beautiful in the spring when the new needles come out. It also has fall color when the old needles turn color. I would imagine that Belgium has similar species of trees that do well.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.