He's not so sure how to address this with a five and eight year old. Trunks will probably get it but Bulla probably won't. Well then.

"What she says is true. This is your sister, Bulla. Let me put it this way- this place we're in now disregards any sort of logic when it comes to time, so you won't know because she simply hasn't been born yet from when you remember." He sighs, this is going to be so much more hassle than he expected.