Yes Jassi, It is an awful thought, but definitely a consideration.
There is an outlet in PDL well away from the beach, it seems.

I'm fairly sure that square cut channel is the outlet just to the west of the Bull/Fortaleza. That's the bright green dot on the left of my graphic.
As you can see, there is a small beach at that location. As it happens, 19 times out of 20 the storm drain empties onto the beach, not into the sea. It is only at extreme high tides that the little beach vanishes under water. (Most high tides leave a little bit of the little beach exposed.) This is not a strong contender for body disposal.
The green dot in the middle is the outlet to the south of the Fortaleza. It empties onto 20 to 30 metres of rocks, so it is definitely no good for body disposal.
The green dot to the east is around where I've read there is another outlet, but it is on a cliff so I'm guessing at precise position. I am far from certain so please don't quote me, but I believe it empties onto rocks at the bottom of the cliff. Whether is does or not, it would require unusual tide and wind conditions to wash away from the main beach.
Anna posted a photo earlier of a channel/stream on the west end of the promenade. That is connected to rainwater disposal also, but as I pointed out earlier, it normally empties into a large depression in the beach, forming a large pond. Sometimes the sea does connect to the pond, but I can't see remains floating over the depression, into the sea, and then away from the beach.
In summary, I don't see the drains as a 'good' solution (apologies) for Madeleine getting into one herself by accident.