As per Wikipedia the overdose threshold for adults can be up to 50k/day https://en.wikipedia.org/wiki/Vitamin_D#Excess

As an anecdote, about half a year ago my sister-in-law's 4 year old found some vitamin D pills and ingested what was left in the pack, in total around 60k IU, hours before anyone noticed.

The doctor said to just avoid giving him any other Vitamin D for the next few months and it should have no consequences.

Luckily it was just Vitamin D, pretty much any other pills she had would have probably been deadly at such a high dose.