Don't worry! Seriously. You should have training from the child's diabetic specialist nurse and should not be expected to do the injections until you are happy.
Ask for the diabetic specialist nurse to come in to work with you. There is not too much that could go wrong (famous last words), and what you are doing will greatly improve the child's quality of life and long term health expectations.
You should be given guidance on dosage, I would request that the parents give you a written form with the amount of insulin that you are expected to give before the meal, there will be a minor adjustment + or - a unit depending on the blood test results before the injection. The written 'protocol' should tell you exactly what to do - you should not be responsible for dosages, but can follow written guidance.
It really is easy to do once you've done it a couple of times.
Pumps are the 'ideal' for diabetes management, however there are a lot of issues around getting them set up (not least the funding issue). It may be the choice that the child's parents take in future.
We are in a dreadful system at present, which ignores the needs of children who require life-saving medical intervention which will affect their long term health. The schools are asked to support the families, however there is no central or county funding for this which would provide an extra 'layer' of funding (ie a bursary for a couple of people to be trained to administer, making it a 'duty'), there is no funding for 1 to 1 support for a newly diagnosed child. Parents of diabetic children are wholly dependent on a sympathetic and understanding staff, with the support of the specialist nurse.
We'll look back in horror in a decade or so (as professionals and parents). We wouldn't tip a wheelchair bound child out of their wheelchair at the door. We wouldn't deny a child their inhaler and we wouldn't refuse to administer an epipen for anaphalaxis.
Get the training and then you will be in an informed position to decide.