1)Arguments that make this point null.
1) Never in canon has it said that they are 2 young kids. They have only EVER appeared in one canon game, and you could say, with logic, that they are as big as a normal human when comparing them with polar bears.
They are 3/5 as big as a male polar bear, which are 11 ft high when standing.
That's 3.35 meters high.
3/5 is 2 meters high. That's as big as ganon.
If that is not enoguh and you argue that they look like kids in Brawl, so they MUST be kids, there is this:
http://tvtropes.org/pmwiki/pmwiki.php/Main/ArtisticAge
2) Peach's grenades (in fact, primitive grebades) were made specifically to burn ships and the like. They are VERY innefective against infantry. The only way they could kill if it the climbers caught on fire and did nothing about it.
And I already proved that Peach's grenades are primitive bombs.
3) Their bad proportions:
That's their coats.
Even if their canon weapons suck, they could still take a princess in a frilly dress with no other protection than really ****y grenades and a parasol.
Thats a lady with a parasol and some fire works trying to fight against 2 mexican muggers.
Ice Climbers win.