We created a play area for our son 30 years ago. Our house is pre 1st war and the joists in the loft were strong enough to cope, so we enlarged the loft hatch and added a strong 2 sectional extending ladder which we attached to the loft floor it had hand rails at both sides. We boarded out the loft to right under the eaves using plywood which our local B & Q cut into narrow widths to fit through the loft hatch ...( other DIY stores are available
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The loft has the same footprint as the house and we boarded up the areas under the eaves to create storage. When it was finished my MIL donated a large carpet. Our Axminster clad loft, once again, has a 60ft train track in place this time for our grandson's enjoyment. Although he once came downstairs to watch cartoons without his dad and grandfather realising they had been playing, errrrr I mean redesigning the layout, with out him for over an hour before they noticed.
The fiddliest thing was sorting out the lighting and putting a couple air vents in the roof as with the roof insulated it can get extremely hot up there and sweaty feet aromas are something we didn't want to have to live with.