Sure I will, all you need is a rope, some nails if you are making proper seat. Then a board of wood with fine cutt. And cutting tools and rest necessary things that are needed in making any wooden stuff. Now cut a piece from the board of 3-4 feet and cut a round circle on both sides of this board or you can make them on all four sides to make it more secure for your grandchild. If you want to make back and arms as well then cut another piece for the back of the seat and a long piece for the arms, more two pieces for the handles of the hands. You can also fix second hand pillows to make your seat comfortable for you grandson.
If you want a clean board as it is then, now take rope as long as it reaches to the trees hand easily and then make it two long strings and knot it with the board and hold two sides of left side and take equal distance from the base of the board so it will remain straight and make a tight knot, do the same thing on the other side and then tie it up with the tree. Take you grand son and ask him to sit on it, if it is perfect then put white glue on all the knots of the rope so it will be a solid knot and your swinging chair is ready.
If you want a clean board as it is then, now take rope as long as it reaches to the trees hand easily and then make it two long strings and knot it with the board and hold two sides of left side and take equal distance from the base of the board so it will remain straight and make a tight knot, do the same thing on the other side and then tie it up with the tree. Take you grand son and ask him to sit on it, if it is perfect then put white glue on all the knots of the rope so it will be a solid knot and your swinging chair is ready.