My daughter is 9 and I got her three pet female mice recently. I am the one cleaning the cage, and don't think once a week is enough. I am cleaning the cage every other day, minimum. We live in an apartment and so there is no "out of the way" place to keep them and so they need to smell OK. The good thing is the cages available now come apart easily and clean up easily.
They are definitely cute and are getting more friendly all the time. She has started taking them into our bathtub for playtime, since it is less likely they will be able to climb and get away. This is making them comfortable with us and they will take food from our hands and climb on our hands now. She has plastic tubes that she can rearrange to make changeable climbing structures for them.
I would be concerned that a 3 and 5 year-old wouldn't be able to handle a critter this small without the possibility of crushing it accidently. They also poop all.the.time. After she has been in the tub playing with them, I have to wash the tub and she takes a shower.
I suppose mice would be OK, if you plan to not take them out of their cage, maybe only into a roller ball, and plan to supervise at all times.
We also have hermit crabs. They are more like a science project than an actual "pet". My daughter enjoys having them because they are interesting, and she will take them out and have them climb on her. The downside is that they molt fairly often. When they molt, they bury themselves in the sand and stay there for about three weeks. Again, interesting creatures, but not really what I think of as a pet. Also, their environment needs to be kept in a temp and humidity range, they need food and water daily, but we do a complete clean of their terrarium only every few months (when we catch all three of them between moltings!).