"Miresand" is in the indicative mood. It expresses that something regularly takes place: The children start exercising when they get to the gym. "Beresand" is in the subjunctive mood. It expresses the (high) likelihood of something taking place in the (near) future: The children are (most probably) going to exercise when they get to the gym. The two can be used more or less interchangeably here.

As the children reach the gym, they exercise. = > Their choice, they came to the gym for that. When the kids reach the gym, they exercise. => It's "required", they don't have the choice.

2nd sentence is in continuous with its former sentences, like the answer to this question that: what will kids do when they arrive at the stadium. but the 1st sentence is as news.

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"miresand" is a certain state "beresand" is doubtfully or hopefully to happen beside, in this sentences "miresand" refers to they habit and schedule but "beresand" almost refer to this time. even though there is no significant difference and you can use both.