We often assume that once children have gained some confidence with small numbers that they automatically can move on to using larger numbers. This may well not be the case.
Here is a simple maths worksheet which is all about counting on single digits from a 2-digit number. The most obvious way to do this is by counting on in ones from the first number, but the question remains as how do you know how many you have counted on? Usually children will use their fingers. For example count on 6 from 24. Start at 24, count one to 25 and hold thumb out, then 26 for one finger and so on. When one hand and one further finger has been extended then they know that they have counted on 6. Essential to this process is knowing how many fingers you need to count on 6 without actually counting out loud up to 6 (as you are counting out loud from 24 to 30).
Adults presume knowledge of this, but it might be worth checking before starting such an exercise.