The number 10000 can be split into 100 + 0 + 0 = 100, which is sqrt(10000).

That's rather banal, but there are two other such 5-digit numbers.

One is 55225:

5 + 5 + 225 = 235 = sqrt(55225)

What is the other one?

Each digit must be used exactly once, in proper order.

Bonus:

How many such 6-digit squares can you find?