Play platform in sons bedroom

[cupcake]

Hi
New here so bear with me!
We [hubby and I] are planning to put a platform in our sons room so he can, hide, play etc. [not for sleeping] It will go above his wardrobe area and up into the vaulted ceiling. Floor space about 4' x 7' - accessed by a rope ladder.
Is this something that we could do as amateur DIYers or not?
Anyone done this already? Any suggestion?

 

[big-all]

picture please

frame off 4x2" timbers and 12mm ply /mdf with battons off 3x2" attached to walls with 3x2" legs
main frame supported[legs] 3.5 to 4 ft on open sides
'3x2" every 19" along the 7 ft [3 cross bits]underneath to support base

legs extend 16 to 18 inches above the deck for handrail with step ladder made from 4x2" planed timber with 4x1 planed treads extending to support the handrail ends
handrail supported every 16"or so 'also handrail height assumes no one can go higher than kneeling