The bridge should be centered between the inside notches of the “F holes.” If it is not, the instrument probably got dropped or bumped. As a general rule, bridges on the smaller instruments slide out of place more often than those on larger instruments. (Perhaps the smaller kids bump their instruments more often – a reasonable assumption! Also, less tension is required on the shorter strings to obtain the pitch, so the strings are not pressing as hard on the bridge.) Another cause of the bridge shifting to one side or the other is a completely loose G or E string. If all the tension is removed from one outside edge of the bridge, it may shift in that direction (as it “tries” to center the three remaining strings).
To determine if the bridge is off center, hold the instrument with the tailpiece at your belly and the scroll pointing away from you. Look at the strings. Are they centered over the fingerboard? If there’s the same amount of black space on each side, it should be fine.
strings off center
strings centered over fingerboard
If the strings have moved one way or the other, hold the instrument between your knees with the scroll pointed downward at an angle. Now grasp the bridge below the strings with the thumb and index finger of both hands. Move the bridge sideways until the strings are centered over the fingerboard. That should do it!
If this needs to be adjusted, hold the instrument between your knees as before, and hold (firmly!) the bridge with the thumb and longest finger of each hand. Place your index fingers between the strings on the top, front edge of the bridge.
Hold like this to adjust angle of bridge
With the bridge held firmly, tilt it a little at a time, till it sits in the proper position. This will take a fair amount of force on a full size fiddle, less on a smaller one.
If the feet of the bridge are not seated completely on the top of the instrument when the bridge is in this position, or if the wood of the bridge appears to be bending, it’s time for a new bridge.
Another sign that a new bridge is needed is deep grooves where the strings cross the bridge. These grooves should be between a third and a half the string diameter in depth – no more.