Forgot to mention that part in front of stairs is a baby gate. Just looking to replace the long end.

You will not find anything premade. The box stores have all these materials (I would not purchase there but that may be just me).

Are you going to change the stair railing as well?

Maybe just me, but, if I had small children, I would be putting a complete new wall there, rather than any kind of railing. Those railings can be very dangerous, if you have anyone that wants to climb them.

I would not go with a wall although it would be somewhat safer for the kiddos; it's just... with all those doors on the second level it will feel like a hotel instead of a home as you walk down the hallway.

I think what you want for a handrail has to be custom made, but easily done by a competent tradesman.

For safety sake, the advice about following code is prudent; getting a child's head stuck at any age would be tragic. On those same lines, you might consider upgrading your light too. It looks like a single bulb and it would be much safer if at night there was more light on those steps? Maybe not so much for you and the kids at your ages, but for Grandma visiting the grandchild? I am 48 years old, but find I require more light now as the years creep up on me.

Yep, it sure sucks being the same age as old people now! :-)