Is it possible for a toddlers blood test to be negative for celiac disease but upon endoscopy or colonoscopy be found to indeed have celiac disease?

Possibly. If the correct test was done (measurement of IgA antibody to human recombinant TTG is recommended for initial testing for CD by NASPGHAN) and was negative, AND if a child has chronic diarrhea or failure to thrive OR if they have selective IgA deficiency or a + family hx of CD, a biopsy might be recommended. Consult with a pediatric GI physician if this is the case. Best of luck!
Celiac. If you specify the test ordered I can answer you better. There are tests more appropriate for a toddler than an adult and some tests such as endomysial antibody is dependent on lab interpretation may not be appropriate to obtain. Why did they do endoscopy if Celiac panel was reported negative ? Was there other indications for it ? Is there a strong family history of Celiac or autoimmune disease?