Wow- a lot of good questions and I hope you have a preceptor who is also able to assist. Before offering solutions/answers, you have raised some important questions that need to be addressed first:
Has the child had speech/swallow consult to determine ability to swallow safely?
Who ordered the corn starch for use as thickener? There are commercially prepared thickeners which allow you to thicken formula to specific thickness. (Nectar, honey, semi-solid etc thickness) A dietician should be involved.
What is the goal of oral feeding? For comfort/oral satisfaction or to try to wean from tube feed to oral nutrition?
As the child has CP, I would imagine physical therapy, occupational therapy etc etc are involved and so should speech and swallow.
Re: timing and quantity of offering oral feeds, ideally oral should be offered when the child's stomach would be least full (just prior to next G-tube feeding) and during the day when the child is more awake.
Hopefully this helps you prioritize actions and who to consult- good luck in your new journey, don't stop asking questions and seeking guidance from those you work with.
Jessie Brodbeck, RN, MSN, AOCNS (former pedi oncology nurse)