「と」 is used as a particle in both cases.
1.「ふりむくと、こどもたちが おおきな やまを つくっています。」
Here, the 「と」 is a conjunctive particle meaning "when" as in "When I did A, I saw B happening."
The sentence means "When I turned around, the kids were making a huge mountain."
The tense used in the original is the present, but it is the historical present, which is why I used the past tense in my translation.
The 「っと」 here is the quotative particle, believe it or not. The speaker is quoting himself, so to speak.
「Verb in volitional form + っと」 is a very common structure used in colloquial conversations. It is a way of making a light and casual declaration (more to oneself than to others). With this 「っと」, the speaker is often just thinking out loud.
"Alright, let's dig a tunnel here!" (Said more to oneself than to another.)