When I play, it is not unusual for kids to ask "can I try?" When possible, I give them a short lesson on getting sound out or at least advice on "playing" water bottles as practice. This year, I noticed that when I play my (new) wood penny-whistle, kids are even more likely to run up to their parents and say "I want to learn that!" So I thought I'd write up some advice about getting a penny-whistle for a beginner.
First, DON'T tell them it will be easy! Getting sound out of a whistle isn't difficult, true, but playing WELL takes quite a bit of work. It requires precise breath and finger control which takes time and patience to develop just like all wind instruments.
Whistles are generally high and loud. Add to that that most beginners tend to blow too hard and beginning whistlers can create some painfully high sounds. It is not uncommon for parents and roommates to get frustrated with beginners who haven't learned to play gently yet. Be patient and/or set up a space where they can practice without disturbing others. Outside on a nice day is great. The lack of walls really eases the intensity of the sound a lot and suddenly the whistle becomes fun to listen to. Putting one empty room between the beginner and others can help too and is a great solution for cold weather.
Be prepared, cats and dogs will likely have an opinion about the whistle. They may hate it and flee the room or they may insist on being right there to keep an eye on things. (I have had a cat who sat in my lap, a cat who hid under the sofa cushions, a dog who howled unless let out and a dog who scratched at the door to sit under the music stand when I played.)
And remember the more the beginner practices, the more control they will have and the easier it is to listen to them in a confined space so don't throw them out of the house forever! You will miss hearing the music they are making as they progress.
Second, if you are getting a whistle for a young kid, I'd suggest getting a C whistle. Most whistle experts recommend D whistles for beginners because they are used most often in Celtic ensembles. But the C whistle is just a little less high and shrill which makes it just a little easier to live with. If they keep playing for more than a month or so, you may want to get a D so they have a more standard key but starting with a C can make the difference in a feud over practice sessions being too high and loud.
For older kids (over 8 or so) and adults, the D whistle is likely a better place to start. There is more music available and as I said, it will fit in with ensembles more often.
Keep in mind that some brands or styles of whistles are actually quieter or gentler sounding than others so it is well worth exploring different whistles (see next tip).
Third, get advice on which whistle to get instead of just getting the cheapest available. One of the nice things about whistles is that they tend to be less expensive than other instruments but all the same, you want an instrument that is well tuned and the player likes to play. Not all whistles fit the bill. There are "tweaked" versions of cheap whistles which can be a good starting point and stores specializing in whistles often have good advice on brands to start out.
Personally, if I was going to get a metal whistle (to go with my wood one), I'd pick a Tilbury Whistle which currently runs about $75. It has a tone I like, is well tuned and while it is considered high end in the whistle world, the price isn't outrageous (compared to some of my other flutes that is). If that is more than you can stand, don't give up the idea. There are $20 or $30 whistles that sound just fine and some excellent players choose to play the cheaper whistles for their entire career.
The Wandering Whistler has reviewed a number of whistles and sometimes includes a sound clip. The recording quality is variable but it can help a lot to hear the whistle played by a good player. Click the drop-down menu to see what whistles he has written about.
Getting a whistle that the player enjoys increases the odds that they will keep practicing and have a good time.
Fourth, get a book of simple tunes for the whistle and/or get some recordings of whistle music. Celtic is the easiest to find but there are other styles of music that the whistle fits into nicely. You want tunes that fit the range of the whistle (about two octaves) and don't include too many chromatics especially at first. The whistle can play chromatics but the cross-fingering and half-holing is more advanced skill-wise so stick with fewer chromatics to avoid frustration. There are actually quite a few books out there so look around and see what looks good.
Finally, if the whistler starts to lose interest in playing after a few months,
take a look at some OTHER whistles. It may well be that they are just
developing a dislike of the tone of their whistle!
Finding a whistle they like better may re-ignite interest. And they will have a better idea of what they like in whistle after playing for a bit.
And whatever you do, have fun!