Hi Popsey
Well, it is up to you, but I don't think this is naughty behaviour - I think they probably get quite a lot out of using their hands to feed, such as:
1. being able to eat quicker than using utensils
2. feeling the food in their hands therefore satisfying a sensory / touching need
3. showing you that they are nearing the end of their meal
It is certainly good to help establish table manners, but 19 months is still relatively young for perfect manners. When they are using their utensils I would give them as much help and praise as possible. When they get fed up and use their fingers, just let them get on with it. Once they start messing around with the food (i.e. not really eating it anymore) they are showing you that they have had enough to eat / need to move on to another activity. There are lots of foods that us adults eat with fingers too - you might want to encourage this by serving a bowl of food to eat with a sppon, then as soon as they start to move into using their fingers, offer some actual finger foods (sandwiches or whatever) to help them learn that some foods are eaten with utensils, some by fingers.
There will come a point in the future where they will no longer want you to help them and they will want to do everything themselves, including using a sharp knive that you don't want them to, believe me LOL
xx
