This is the simple height adjustable chair to complete my kid furniture set. You can find the instructable for the desk here https://www.instructables.com/Kid-Desk-height-Adju...

The idea to increase the height when the kid grows to give them the best comfortness. You can find the instruction video here https://youtu.be/el8zthcq9Jw

This time, I added the Fusion 360 file so you can modify the design to fit you better for examle the size of the seat or back support.

In case you want to know how to design it in Fusion 360, you can have a look at my tutorial here

Supplies

Materials

wood as in the cut list
8x wood inserts
8x bolts
wood screws
wood glue
sanding paper
stain or varnish (optional)
Kreg screws

Tools

miter saw
drill
Kreg pocket hole jig
ruler
sander (optional)

Step 1: Gather Your Wood

I attached the cutlist for the chair I made. In the photo, you can see the size of the chair compared to a teddy bear :). Each chair needs:

2x 2inx2inx8ft
1x 8inx48x0.75in board

The only difference is that instead of using a 16x13 inches board for the seat I use 2 pieces of 8x13 inches. Do it this way I need only one board of 8x48 inches for the seat and the back support.

In case you want to tune the size (blue text dimensions), feel free to edit the parameters in the FUSION 360 file (FUSION 360 -> Design -> Modify -> Change Parameters)

Step 2: Build the Chair Frame

For each frame of the chair you need

1x frame long
1x frame short
2x horizontal bar

The most important task is to drill holes for height adjustment. Sizes and distances are in the photos (you can change it according to your hardware). You can drill 2 bars at a time to be consistent. Then, you can use wood screws to attach the horizontal bars.

Step 3: Assemble the Frames

The seat bars will carry the seat up and down for changing the height. You can drill holes and put the wood inserts in the two sides of the seat bar for that purpose.

Step 4: Prepare the Back Support

The same as seat bar, you will need 4 inserts for the back support. Pay attention to the size and distance between holes, make sure that they are agree with the holes on chair frames.

Step 5: Build the Seat

If you use a board of 16x13in for the seat, skip this step. In case you use narrow board for the seat, you will need to attach them together using Kreg jig pocket.