This is a fairly basic tutorial for padding a mask to fit a smaller wearer. This is primarily for resin masks, but the same principle would apply to foam masks as well. Pretty much anyone can do these simple fixes to their heads if needed, in the range of $10.
You may additionally need padding in the top/back of the head, and you may need to adjust the jaw's motion due to the change in pressure points. This is just a basic starting point.