A toddler's food intake seems very little: only milk and water and some solid food. Therefore, from where does this energy come?
They don't eat little. Consider the volume of milk / food they consume as a ratio of their weight. Quite the contrary they are ravenous machines and their consumption is much higher than an average adult. All cellular energy comes from the hydrolysis of ATP, and the production of ATP comes from the breakdown of glucose. The glucose comes from a sugar found in breast milk, called lactose.
Just to add to the answer from @buzrw:
Using data from here and here I have estimated recommended calorie intake per kg for a 2 yr old and for an adult. I've used figures for males and assumed 70 kg for the adult weight which is the "international standard man":
toddler: 120 Cal kg-1
adult: 40 Cal kg-1
So yes, the energy intake of toddlers is higher than that of an adult.