Depending on what your 3 yr old did, the most likely was to select all and click on Treatment, Grayscale...if that's what happened, select all and in the Library, under Saved Preset, Treatment, select Color...
Otherwise, you may be in a situation where you want to revert to a previous catalog backup or the odious method of going through the images in the History in Develop and select a state prior to the conversion to grayscale...
And I might add that in the future, keep your 3 yr old away from the computer...get the kid an iPad!!!