You have to use words in novel and creative ways if you want to truly speak the language. For instance, you cannot really make jokes in other languages unless you twist the meanings of words by using puns, double entendres, innuendo etc., so it's always good to know various meanings of words, and to try and make sense of phrases that are illogical, because the fact is that people are tricky all over the world (and looking like a fool is universal).
Historical note : it's because, traditionally, the elder son of the king of France would be given the duchy of "Dauphiné" (inhabitants : "Dauphinois"), in the South East of France, so the duc (which was almost always the first heir in line), was called "Dauphin" by extension, or abuse of language.
It makes a lot of sense for most children to say I am a dolphin. There are only a very few people in the world where it makes sense for them to say I am an heir apparent and even they would usually use more common, less technical terms to express that thought. eg: prince. (the difference between the prince and just a prince is lost on most people most of the time)
However you are right, in the strengthen skills section you probably lose the advantage of having a connection to the most likely, general sense of the word.
The thing is, even if someone wanted to say "I am the heir apparent" in French, he would say Je suis le dauphin de France or even without the article : Je suis dauphin de France. So, as a native French speaker, I would never think of interpreting this sentence as meaning "heir" instead of "dolphin".
Just because some meaning of a word in a language got adopted into another doesn't mean it's the most used meaning of that word it its original language!
I think very few children pretend to be the heir apparent although some might pretend to be a prince. (or princess)
My grand daughter prefers to pretend to be animals rather than princess. I have seen her pretend to be every animal you can imagine including dolphin. I have never seen her pretend to be a princess. When she was in her Barbie phase she played many roles with them but never princess. It just doesn't seem to be as popular as it once was.
I think most heirs apparent would be likely to describe themselves in their own language rather than use the French term since there is no French heir apparent anymore.
Not sure what French children think but in North America the dolphin is regarded as a cutie pie kind of animal and attracts the interest of a great many children. The anecdotal evidence provided by my own limited play acting with young children is that they describe themselves to me as animals in general and dolphins in particular much more often than wanting to role play as a prince or princess.
I find this surprising since I am a monarchist myself.
I grew up (and learned French) in Mobile, AL, USA, which was a French colony for a century or so, predating New Orleans by decades. Among the thousands of French place names in south Alabama, Mississippi, and Louisiana, there are Dauphin named things and places all over, Dauphin Island being probably the largest and most important. I never knew there was any definition but the Prince one! So I got this question wrong, and was very confused! I'm curious how a section of France came to be named after a sea creature, and how in a place where you can actually see plenty of dolphin, I never heard the correct definition. How language travels and leaves pockets of other cultures flung all over is so fascinating!