Well, first of all, you're talking about a major time-warp here. In a fight between baby DK and baby mario/Yoshi, I would say definitely babymario/yoshi. Full grown DK versus baby mario/yoshi...I'm afraid I'm gong to have to say DK.
--Considering that in Yoshi's Island the slightest touch would cause baby Mario to cry his eyes out and float away, limiting Yoshi's lifespan to about 30 seconds at most, that really wouldn't help against a giant monkey with a huge armspan
--baby Mario has no experience or wisdom. We all know that with the exception of Star Mario, Yoshi did all the work. Yoshi is a green dinosaur that ends up getting caught by Bowser's children in Super Mario World and needed saving by Mario, so we know he can't fend for himself against anything big and ugly.
--DK, although big and clumsy, packs a whallop. He'd knock Mario off Yoshi's back so fast, it'll make both their heads spin. I doubt that Yoshi's tongue attack would do much good. DK would probably grab the tongue or the taste of DK would make Yoshi do that cryface thing.
