Yummy just means 'tastes delicious'. Yum, is a childish non-word, but adults use it occasionally. We might say 'Yum!' as an exclamation upon seeing some delicious food. Yummy can be used as an adjective - 'That was yummy, thanks!' Yummy in my tummy is a pleasing and childish rhyme. It has no specific meaning, other to express excitement about eating something delicious.

'Yummy' is a childish and informal word used to describe something that tastes nice. It's a made-up word derived from the sound 'yum', which is a non-verbal expression of approval about tasty food or drink. The word is used to and by children, and sometimes by adults - mainly young women. Older people use it less, and I have never heard a man use the word 'yummy'. It's sometimes used in advertising, especially for foods and drinks aimed at children and young women. Of the three phrases you suggest, I've only heard 'Yummy in my tummy'. 'Tummy' is an informal and slightly childish word meaning stomach, and the two words are obviously put together for the rhyme. And to answer your question, yes, it does sound childish. Unless you're actually trying to sound cute and baby-like, I wouldn't recommend saying 'Yummy in my tummy'. It isn't a normal thing for an adult to say. You can use the word 'yummy', if you like, among your family and friends, but not in more formal situations. For example, it would be odd to use the word to a waiter or waitress in a restaurant - in this situation, you'd use a more neutral word such as lovely or delicious.

'Yummy' as an adjective means 'delicious.' You can say 'these/those sandwiches are yummy', but it is a word that is most commonly used by small children, or by adults around small children. So that is what is meant when people say it sounds childish. 'Yummy in my tummy' means something you've eaten tastes really nice. Yummy = delicious. Tummy = an informal way of saying stomach, again commonly used by children. They are things that small children say, or that adults might say to small children. I don't know why, I guess because they tend to like things that rhyme. It would be weird for adults to speak to each other or describe things using this language.