3yo is a bit young, Local RSPCA won't let you have a kitten under 6 months if you have a preschool child in the house. 3 months ago, From CPL, we got 2x4.5 month old kittens before youngest DC was 4yo luckily our kittens are like rag dolls & have taken well to a bit of pulling & funny holding (not cruel, honest, but not ideal either). Hard to find an older cat rated as suitable for young children, so you probably have to get a kitten age 6-12 months, still malleable & tolerant.
We got 2 kittens to reduce the child load on each kitten. Two is 4x as fun as one, though, being litter mates they live together well & are very cute together. Lots of funny play fighting.
Costs: Combined £100 in adoption fees & one off equipment (carriers), £7/week each in other running costs, which should go down to more like £5/wk in time (no more big medical until elderly I hope, and they now mostly toilet outside).
Worming 3-4x/yr depending on hunting skill, said vet.
Flea control in warm months at least, this was my huge fear, I dread bites.
Their poo makes the bin STINK. But probably no worse than a dog could.
Mine are not insured, it's not cost effective on average/statistically.
Furniture... hum. They are half good about using the extensive cat tree scratching area DH created, still go for sofas & floor carpet a bit.
We manage without cat flap, I leave them out in good weather/day time. Ours kept in at night, removes a large number of problems! No "presents" brought home yet. Only significant problem is neighbour dog trying to destroy his fence to get at them. (sigh)
They are disgusting in lots of ways, entirely instinctive, and very very stupid. But cuddly & low maintenance otherwise.