When building a children's playhouse, it needs to be a safe structure. You want it to be as sturdy and secure as your own home. You should follow the same procedure for building any other house, just on a smaller scale. For the average age and size of children interested in a playhouse, a house 4-by-4-feet square and about 5 feet high is a good size playhouse to build.
You need a sturdy foundation on level ground. Dig away the dirt where it's needed, lay sand around the area, place 4 cement blocks to form a 4-by-4-foot square and add 4 more blocks between those corner blocks. Use string to make sure the blocks are level and aligned. Now build the floor rim with pressure-treated lumber, cutting four 4-foot-long boards and nailing them into a square. The floor joists are nailed within the rim, 16 inches apart. Lay the rim onto the block foundation and glue, nail or staple roofing felt to the joists. Add the floorboards to the rim, attaching each floorboard to each joist. Instead of nails, use galvanised rustproof screwshere.
Build the 4 wall frames the same way as the floor rim, making them 4 feet long and 5 feet high. The "joists" in the walls are called "studs," which should again be placed 16 inches apart. On the front wall, however, leave the correct open space for the door, placing two studs at the distance equal to the door's width and attach a side board at the door's height. For any windows, space the studs for the window's width and attach side boards for the window's top and bottom. Screw each wall to the house's floor and then screw the walls together. Create the wall siding with T111 panelling or "sheathing," cutting it to the walls' size and cutting out the holes for the windows, then nail the sheathing to the house's sides. Don't create the siding for the front and back yet.
To make the roof trusses, cut and attach wood so it can form an even triangle when placed on top of the wall. Add the "gusset" to each truss by cutting out and screwing a small piece of roof sheathing to the tip of the truss' angle. Make 4 such trusses and attach them to the top of the house, putting one each atop the front and back walls and the others along the side walls equidistant from each other; screw them into the wall frames from the underside. Attach wood pieces between the trusses for more support; create a line of boards that runs along each slope of the trusses. Now cut the siding for the front and back walls so they include the added triangle shape of the trusses (and the opening for the front door) and nail them to these walls. Finish the roof by nailing roof sheathing atop the trusses, stapling roofing felt to the roof and nailing shingles to the felt. Attach the door to the front door frame with hinges.