A diet of rice only is not likely to be adequate, oats, wheat and maize in addition would help and in sufficient quantity should suffice. If the bird succeeds in hatching one or more eggs it will, if sufficiently fed, feed the offspring by expressing what is called crop milk. Thus so long as it has sufficient food and water it will survive and raise its offspring. If you want to try and have a go as a protector to the birds then you might do well to create a cage for them in situ where they are, this would then serve as a safe haven - predators are a very real threat, but you would have to ensure that sufficient food and water are provided. One point, if there is a risk of the parent bird struggling to keep the egg and/or later the offspring warm on the ground, for example if it rains and the ground becomes waterlogged and cold, then it might be worth attempting to put something underneath the egg/infant to raise it and keep it dry/incubated/protected. Perhaps you have a small, flat basketry item of some sort ? If the bird stops sitting on the egg then it is all over.