Looking good!

Although there is one thing I would have done different: ventilation for the mattress is missing!

Hi,

Every mattress manufacturer requires proper support and ventilation of their products , otherwise they void the warranty. You got the support right. But there is no ventilation happening. Due to temperature differences and body moisture every mattress will, to some degree, get "wet" on the underside. In your case I would recommend drilling at set of 2" wholes into the platform to allow for some type of airflow. Not ideal, but could do the trick.

Here is how we are doing it: http://www.ecofurniture.ca/econews/archives/category/andreas 

Cheers,
Andreas

Mattress companies seem to have their warranties written by a team of evil lawyers. If you look at a mattress cross-eyed, you will void its warranty. (I say this from experience.)

You do need ventilation from the underside. It is very important to prevent bad odours building up inside the mattress.

So drill some holes in it, but not 2'' likes someone said, make them 1/4'' and make a lot of them.

Like in this picture. It's called a bed plate.

Some? They ALL do that. It's standard sales talk. And they do incorporate ventilation holes because it is important. You see, the average human sweats 1/2 liter (1.25 pints) per night. That moisture goes into the mattress, and then has to get out of it again.

But it is not enough to put ventilation holes solely in the mattress. Once the moisture leaves the mattress it also has to leave the bed. But if you use a solid base it can't leave the bed and will slip into the mattress again.

1 - http://www.childrensbedcentres.co.uk/storage-beds/2-drawer-high-storage-bed-1

"Bed consists of a solid bed base (with ventilation holes), two cupboards and two drawers on runners for storage under the bed."
 
2 - http://www.gltc.co.uk/fcp/product/-/gltc_exclusives/Fire-Engine-Toddler-Bed/2217

"Please note: This bed has a solid base with ventilation holes."

3 - https://www.perweek.co.nz/flora-girls-bunk-bed/

"All cilek bunks are unique in their design, featuring, solid bases with ventilation holes (this makes the bunk more stable and makes it healthier for the child on the bottom),"

4 - http://cgi.ebay.com.au/Pink-Fairytale-Princess-4-Poster-Toddler-%2F-Child-Bed_W0QQitemZ250406819817QQcmdZViewItemQQimsxZ20090415?IMSfp=TL090415125001r15850

"This bed has a solid base with ventilation holes."

5 - http://www.victorsteel.com/category.asp?cat=Hospital%20Beds

"Mattress base: 1.2mm steel sheet with ventilation holes."

6 - http://www.ilcaustralia.org/home/search4.asp?state=NSW&page=1&MC=21&MinC=9

"BED CONSTRUCTION
Base and frame
The bed may have a steel or wooden construction. The base may be wooden slats, wooden with ventilation holes, weld mesh or sheet steel with air holes."

7 - http://erniesplace.com/_BoatingDiaries/090100_Jan/Diaries/090116_Banbury_South2.shtm

"Bed linen wash day on Ten Bob Note!! While the bed was stripped I decided to check out the inside of the base for dampness as last winter there were problems with condensation. Was very pleased to find no evidence of any dampness at all. After the problems last year, I added a considerable amount of additional ventilation to the bed base to increase air flow, it has obviously helped."

8 - http://www.stopsidsnow.com/FAQsAboutMattressWrapping.html

"My crib has a solid base and the ventilation holes on the
underside of the BabeSafe cover are obstructed. Do I need to do anything about this?

Yes."

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Really, there's so much about ventilation holes in beds out there on the net.

If you don't believe me, or anybody else here, go to a local bed dealer. Ask them what they think about it.
   
But seriously, I pity your niece if she doesn't get proper ventilation. If she doesn't, it doesn't matter how nice the bed looks (it does), but she will still have a problem.
 

Alex,
Believe me, it's very easy for me to add some holes if they are needed.  And I'm really not trying to be confrontational in any way.  I'm just trying to understand the rationalle for ventilation holes under a mattress.  Here's my thought process.

Yes, people sweat at night, but how much of that actually ends up in the mattress?  I would think that far less than half given the surface area of a body actually in contact with the mattress.  Of that which enters the mattress, why wouldn't most of the moisture escape back through the top and sides, as this surface area is probably 50 to 100 times larger than the amount of bottom hole surface area?  Like you, I've seen a lot of recommendations from frame manufactureres for vent holes, but I can't find any similar recommendation from mattress manufacturers.  Further, what happens to the moisture from those mattresses which are non-breathable, such as latex or waterbed? 

Assuming the mattress manufacturers also recommend ventilation (I've emailed a couple of them to ask the question),  how would a bed frame designer determine how much ventilation would be enough?  I couldn't find, nor have I heard of any formula or rule-of-thumb guiding the decision.  Absent any ventilation sizing data, is it possible that most, if not all, of the claims made by frame manufacturers  of providing adaquate ventilation are merely sales hype?

Thanks for your concern over my niece's health.  It's making me take this issue serious and give it the proper consideration.

We've had a platform bed (mattress only, no box springs) for 30+ years and it does not have ventilation.  The base it sits in is solid and the sides come up about 3 - 4 inches all around.  No problems!  Don't worry about it.  Remember to rotate the mattress once in awhile to keep the wear pattern changing.  I'd actually be more worried about spiders, mites, etc getting into the mattress from below than about ventilation.