We haven't even moved in and have already changed our minds.... 
Currently, we have a straight steep loft ladder with short step depth (and closed steps) leading up to a small attic floor. It has now become clear that we'll want to place one of the children up there in a few years since more children are in the planning stage. Then the ladder seems a bit dangerous for daily running with children's legs. I've come up with an idea to increase the steps by 50% using MDF and 45-reglar, and to swing the last steps out in the hall below in the form of a quarter circle.
The question is whether the ladder can handle carrying a bit more wood of this sort without needing reinforcement in some way. I imagine it will be fine, but it's best to check here first
Currently, we have a straight steep loft ladder with short step depth (and closed steps) leading up to a small attic floor. It has now become clear that we'll want to place one of the children up there in a few years since more children are in the planning stage. Then the ladder seems a bit dangerous for daily running with children's legs. I've come up with an idea to increase the steps by 50% using MDF and 45-reglar, and to swing the last steps out in the hall below in the form of a quarter circle.
The question is whether the ladder can handle carrying a bit more wood of this sort without needing reinforcement in some way. I imagine it will be fine, but it's best to check here first
If the staircase can handle an extension is hard to say without having seen it... and in any case, it's not my strong suit to answer such things. However, I have another thought.
You mention that if you were to install a new staircase, you would need to extend the joist opening - I assume it's to make room for a gentler stair without hitting your head on the edge of the opening when you use it.
But don't you face the same problem if you extend the existing stair, as it would also result in a gentler stair?
And if you have to alter the opening anyway, installing a new stair would probably look better than an extended one that might end up looking quite odd.
Rocket science - it depends. If the current opening is within a joist bay and the stair goes down parallel to the joists, you can extend the opening within the same bay and then you don't need to modify the joists themselves unless there is a trimmer that needs to be moved.
If you need to alter the joists themselves, someone knowledgeable should look at it. It may also require a building notification to change load-bearing structures.
A bit OT: From my experience with our four children (the youngest is 2 1/2), sometimes there's a bit of excessive fear regarding children and stairs. Unless the stair is open to the side, allowing a child to fall straight down, and unless there's stone or tile in or below the stair, small children rarely hurt themselves much when they fall down stairs that have a reasonably normal slope - they are so light and their bodies so supple and flexible. But in this case, it seems to be an unusually steep staircase, which can be another matter.
You mention that if you were to install a new staircase, you would need to extend the joist opening - I assume it's to make room for a gentler stair without hitting your head on the edge of the opening when you use it.
But don't you face the same problem if you extend the existing stair, as it would also result in a gentler stair?
And if you have to alter the opening anyway, installing a new stair would probably look better than an extended one that might end up looking quite odd.
Rocket science - it depends. If the current opening is within a joist bay and the stair goes down parallel to the joists, you can extend the opening within the same bay and then you don't need to modify the joists themselves unless there is a trimmer that needs to be moved.
If you need to alter the joists themselves, someone knowledgeable should look at it. It may also require a building notification to change load-bearing structures.
A bit OT: From my experience with our four children (the youngest is 2 1/2), sometimes there's a bit of excessive fear regarding children and stairs. Unless the stair is open to the side, allowing a child to fall straight down, and unless there's stone or tile in or below the stair, small children rarely hurt themselves much when they fall down stairs that have a reasonably normal slope - they are so light and their bodies so supple and flexible. But in this case, it seems to be an unusually steep staircase, which can be another matter.
I'm going to wrap the boy in question in foam rubber and let him run up and down the stairs for a couple of hours before I continue thinking, the stairs aren't steeper than I can slowly walk down with a two-year-old in my arms without risk, i.e., without holding the handrail, thanks for the input, children adapt quickly too it's worse for someone else
it's worse for someone else