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I was asked for first time to help at a non federated tournament for kids to arbitre their games, played at 30 minutes with no increment time.
It was a scholar tournament and they made mistakes as leaving checkmates in one, let the queen to be captured etc.

At one game both kids arrived to a K+R K+R endgame. There was not a way to quickly gain the rook and win the game and both kids had time.
One kid claimed it was a draw. We were two arbiters (not experts, just two 1900 helping a bit at the club).
We claimed draw.

Was it a correct decission or we should have left the kids play?

It is difficult to say what the exact rules should be if you aren't affiliated with a group that publishes rules for such situations. But I think your actions in this case were reasonable.

USCF rule 14H used to state that a player with less than two minutes on the clock (with no delay or increment being used) could make a claim of insufficient losing chances. Officially, that rule is no longer in effect, although a tournament is still allowed to use it as a variation without announcing it in advance.

If this variation is used, then rule 14I4 specifically states that rook vs rook should be a draw unless it is a position where there is a quick win. In general, if a class C player with ample time would not reasonably lose the position against a master, then the draw claim would be upheld, under this variation. This is regardless of the actual strengths of the players involved or how little time is on the clock.

Since there were more than 2 minutes on the clock ("both kids had time") technically this variation could not have been invoked. However, an attempted draw claim also constitutes a draw offer, which the other player may accept even if the claim is not upheld. According to your comment on another answer, you asked the other player if they agreed with the draw, and they seemed happy with it. That would be good enough for me. If neither player wanted to play on, there would be little point in forcing them to shuffle the pieces around for 50 moves. It's possible that novice players would not even be aware of the possibility of simply offering a draw.

The part of the USCF Official Rules of Chess book which contains the rules is now available online, by the way. I found it here.

It is not a drawn position according to the rules, since there is sufficient mating material. It may be a draw from the point of view of endgame theory, but given players who make lots of mistakes, it wouldn't be all that surprising for one to lose to a tactic.

I would let them play until the player who wanted a draw can claim it based on the 50-move rule or threefold repetition, or someone runs out of time or the game ends some other way. (Were they keeping notation? If not making a proper claim could be difficult, although an arbiter who witnesses the game could rule it a draw after 75 moves or fivefold repetition. Also note that USCF rules allow the arbiter or a deputy to count moves, if requested, for a 50-move claim under sudden death.)

That said, for a "non federated kids tournament", I suppose you may have some leeway, so I wouldn't really fault you for ruling it a draw.

You can declare a draw and in fact you are required to declare a draw but only after you have counted 75 moves by each side without a capture or a pawn move. This is according to the FIDE Laws of Chess article 9.6.2:

9.6 If one or both of the following occur(s) then the game is drawn:

9.6.1 the same position has appeared, as in 9.2.2 at least five times.

9.6.2 any series of at least 75 moves have been made by each player without the movement of any pawn and without any capture. If the last
move resulted in checkmate, that shall take precedence

Of course the players are also free to agree a draw between them.

It's better to let them play it out a bit and ideally only draw once reaching 50 moves. But if one of them is low on time (and their opponent is clearly just trying to flag them) then you should claim the draw.