Are there any rules governing whether or not to use に following a 「~の時」　phrase?

I've just come across 2 sentences in the same practice exercise in Genki 2 that look really similar to me, but only the first one includes に.

１．子供の時に何ができましたか。

２．子供の時、何がしてみたかたですか。

(I see there's a comma in the second sentence, but I thought the use of commas in a simple sentence like this was somewhat subjective?)

I find myself wanting to say things in beginner conversation practice like, "When I lived in Sydney...", "When I was at Uni...", "When I went to Japan..." and I'm never sure whether to use に。

I think you can use either, adding a に might buy you more time when speaking, but the comma is there in the other instance, either way, this is the pause when you get backchanneling in a conversation.