Slowing down mp3 tracks
When first playing along with a piece, it can help to slow down the tracks while you get used to the speed. You can search for mp3 speed changer or try these:
On ipad or iphone, try the anytune app
On andriod, try the audipo app
On a computer, try the VLC player. On this software, the speed selector isn't always visible on the player by default but you can put it on by following these instructions: Go to the Tools menu, then "Customise interface". Now in the "main toolbar" tab you should see a scrollable list of many items that you could put into your toolbar. Scroll right down near the bottom to find the "Speed selector". Click and drag this up into the main toolbar area, then click close. You should now see a little "1.00 x" in the bottom toolbar area of your VLC player.
There are many ways to tune your violin:
You can tune your violin to a piano.
You can carry a chromatic tuner in your case, like the D'Addario or the Snark SN-5 (especially good for a child who doesn't have a phone)
You can use an app that sounds the pitches of the strings (like a tuning fork) or you can use a chromatic tuner app.
For most people a chromatic tuner app on their phone will be the most practical solution. You can search for "chromatic tuner" or try these:
On iOS InsTuner
On Android Best Tuner
On computer Tuner Ninja