My kid (6 years old) is learning the piano. Should she use the easy (simplified) version of a slightly hard piece, or an easy original piece?
I am afraid she will have problems with the original version of the harder one if she gets used to the simplified version. Or am I worrying too much?
If you have the option of an intrinsically easier piece that will teach them the same things, that's definitely a fine choice.
However, yes I do believe that you are worrying too much. Playing different styles and arrangements of the same song is something I've always enjoyed doing, and I think it's actually beneficial. Something about the contrast allows you to notice more about what makes that version unique, and thus express it. So I think the simplified version is a great option.
And one thing you definitely want to avoid is presenting a difficult piece too early. Challenge is good, but too much at once will invariably result in sloppy play — and such bad habits are very hard to break.
In that age it is most important, that your child has fun playing the piece, so it will actually like to play it. There is no special value in the "original" version for a child, and pieces come in lots of variants (instrumentation, transposition, arrangement, ...). I would more expect, that after some time having made technical progress the child will be grateful to find a more demanding (read: less boring) version of the piece, which it likes.
In general, someone with the skill to handle a hard piece of music will be able to learn it regardless of whether or not they've previously learned an easier arrangement. In most cases where someone who has learned an easier version finds a hard version frustrating, the problem is simply that they lack the skill to handle the hard version, and starting with the hard version wouldn't help.
The key think to watch out for is that while some "easy" versions do a very good job of preserving the key musical aspects of the piece (especially rhythm), some versions simplify them so much as to change the fundamental character of the piece. As an extreme (hopefully hypothetical!) example, if a rendition of "Don't mean a thing if you ain't got that swing" were to replace the "doo-wop" section with thirteen equally-spaced notes, someone who got used to that as being how the piece goes might be stuck thinking the actual correct version seemed "weird". This may be countered by ensuring that a person is familiar with how the real version of a piece should sound before they try to play a simplified version and, in some cases, by encouraging the person to play correct rhythms whether or not they are actually notated.
Learning to play music is about developing techniques and learning to sight read and count the rhythm, not really so much about the arrangement. The focus should probably be on the process of learning and not so much how advanced a beginner the child might be. Simple songs are a tool that is used to facilitate learning through repetition and the performance of the song is an easy way to determine how well some one is making progress. When we first send a child or even an uneducated adult off to school, we don't start them in tenth grade because they need to learn the basic alphabet and numbers before they can read and write and add and subtract. Its the same with music. The most important thing a parent can do for a child learning music is be supportive and help the child enjoy their learning processes. If a child enjoys making music they will develop the skills and grow.
Yes, you're worrying too much. Play what you can play well, at your present technical stage.
Though I do wish 'easy' versions of 'Ode to joy' didn't leave out the syncopation. It's not like it's hard to play.
