It's a great place for kids. There is so much that they would enjoy doing, especially the older ones.
Corcovado has a cog-wheel train that goes up the mountain (with a live samba band, to get instruments from to play along); Sugarloaf has a two-stage ski-mountain type cable tram and lots of miko monkeys in the trees at the top; the Lagoa de Freitas has swan paddle boats; Santa Teresa has a streetcar that goes over the 2 story viaduct (but they will probably want to hang off the trolley like the local kids dangerously do, touching their toes to the ground); Tijuca forest has troops of spider monkeys, even in the road; and much more.
There is a little store near the Sambodromo that rents Carnaval costumes to take photos. You can also walk the 'Passarela de Samba' through the Sambodromo. Next door, watch the kids and teens learning the moves for the important position of porta-bandeira (flag-bearer), always a woman, and the mestre-sala (master of ceremonies), a man.
On Sunday, join the carioca families at Quinta de Boa Vista, where there are rides even for the little one, the zoo with many Brazilian animals and a good small ethnographic art museum too.
You might want to think about staying on the Niteroi side where it is a bit more tranquil, and cheaper. There are good beaches on that side and you can enjoy the ferry ride across into Rio like the local commuters.