It's not a pleasant experience to be freezing when you're trying to sleep in the night but it's also not better to be too warm. That's why it's important to choose carefully when buying a sleeping bag and liner if necessary.
The purpose of a sleeping bag is to maintain the body's temperature when you're sleeping, not to warm you up. Body temperature varies from person to person and therefore it's important that you know how your body responds to hot and cold weather before choosing a sleeping bag and perhaps liner.
A liner can be uses inside the sleeping bag when it's particularly cold and can also be used on its own as a substitute for a sleeping bag in warm climate.
When choosing a sleeping bag, another thing to keep in mind is whether the length of the sleeping bag corresponds to your height. For children - check out the Junior sleeping bags.