My students will often hear me say to them, "ok, count yourself in...".
Hang on, what does that even mean? To "count in"?
Well, it's all about the numbers you say before you start playing.
At the beginning of each piece, you have the time signature. This is two numbers, one on top of the other. The top number indicates how many beats are in each bar, and the bottom number indicates the type of beat (4 = crotchet beats, 8 = quaver beats, 2 = minims).
So, if the time signature states 4/4, there are 4 crotchet beats in a bar. So you would count in by saying out loud, "1, 2, 3, 4" at the exact speed at which you want the pulse of the music to go, and then you would start playing in time to that pulse that you have established. If the time signature states 3/4, there are 3 crotchet beats in a bar, and you would count "1, 2, 3".
I also get my students to tap their foot (or tap their toes inside their shoe) along with the counting, and then to continue tapping their foot/toe whilst playing. It's not easy - it's a bit like co-ordinating patting your tummy whilst stroking your head in circles - but it is so worth getting into the habit and well worth the practice.
Another good tip is to actually imagine how the piece goes in your head before you start counting in.
Parents/carers - you can help out here by checking to see if your child is tapping their foot whilst playing, and if you tap along too, you will easily be able to check to see if your child is inadvertently speeding up or slowing down!