I have "kid" tested this with a 4 year old and they could break the bar a few times but I ended up doing most of the work. Thus kids please have an adult help with this activity.
Step 1: How Many Chocolate Bars?
Most chocolate bars I have used are about 3-3.5 oz and recipes will call for 12-16 oz of chocolate chips. So to replace chips one for one to chunks you will need to break up 4-6 bars. I have found one to two bars broken up works find in the recipes I make. (I like them on the lighter side of chocolate.) I normally add chunks until the batter looks "full" or it looks like I will get 4+ small chunks per small spoon dropped cookie.
Step 2: Materials
Plastic bags (gallon freezer bags are less likely to break and will keep this "no-mess")
Large metal spoon**
*Calculated from previous step.
**Plastic will not work well due to the plastic absorbing the force.
Step 3: Freeze the Chocolate
Step 4: Put Into Bag
Do not decide to save the bag and go on to the next step with the chocolate in the silver wrapping. The silver wrapping will break apart and throw chocolate chunks all over and the size of the chunks will be very inconsistent due to no viability through the wrapping.
Step 5: Breaking the Bars
Try to smack the bar in the middle of the area that needs breaking.
If the bar starts to dent or mush then refreeze.
I like my chunks smaller then chocolate chips so I break the pieces more then if some liked large chocolate chunks.
The smaller chunks melt better in recipes, like double boilers.