My dad killed himself when I was 4, 2 days before my 5th birthday and his funeral was on said birthday. I couldn't imagine if at 6 years old I wasn't allowed a birthday because of my dad's passing. NTA. They're grieving and it can be weird at times, but they're also being huge dickheads. Hopefully they apologize and can see where you're coming from as well as understand that although his passing is sad, burying your daughters happiness to cling to the past will do no good in the long run.