First of all advance wishes for your son. Shivasamudra is a beautiful place. I visited this place thrice in one year. Attaching a picture which I taken during my visit this year on May 1st week.
The route is good, you can easily maintain 80KMPH throughout. Only the last approximately three kilometers are average (not bad too). The route is simple
NICE Road - kanakapura road - Malavali - Turn left towards Kollegal, you will have boards for the falls.
Note, there are multiple view points, but don't miss Gaganachukki Barachukki falls. Thats my favorite. You need climb down it is steep and close to 200 steps laid newly. You have coracle ride there. They will take you closer to falls, and it will be fun for you son and family. You can bath in the falls too.
I advice you to bring your food from home. Enjoy your day there.
It is more of a historical place. You have park sort of thing (river bank). You can take a coracle ride too here. Main attractions are temple burried on sand dunes. Try to reach there before dusk, as there are no lights near temple and the park too closes at 6PM, if I'm not wrong.
From Talakad I took a route to Mysore road, though it is a two lane the road condition was excellent. A localite helped me and it was dark so don't remember the way. But there are sign boards. From Mysore road it should be breeze for you till reaching Bangalore.
Enjoy your son's birthday and share us pictures. How about cutting a cake in coracle for birthday!!