Mumsnet would say guinea pigs/rats - at least 2. Guinea pigs need a large enclosure at least 140 cm by 70 cm and they need daily cleaning, daily fresh vegetables and unlimited hay as well as regular floor time. But they are quite easy for little hands to handle so are preferred as children's pets despite being high maintenance in terms of cleaning (they can poop as much as 100 times a day and they pee for england too). Rats are an acquired taste imho but they are very clever and are like pocket dogs in the sense that you can teach them to do tricks and they are very loving.
I prefer a hamster myself as there is less poo/maintenance and you only need 1. As your children are 6, you would be doing the cleaning and I imagine twins are probably a lot of work. they are nocturnal but you can tempt them out to play with treats in the evening and hand tame them. They are just as cute and fluffy as any guinea pig, but smaller and lower maintenance. I have a male syrian , they are slower and easier to handle, and females smell when they go into heat.
Gerbils could be a good idea as they don't smell (being desert animals) and are fun to watch, they also come out in the day. However they are very quick so its harder for children to handle as compared to a syrian hamster, but they are quite fun to watch. However, I was put off by the declanning aspect- gerbils can fall out of love with their cage mate but yet they must be in pairs- so if any fighting occurs, you must separate them and then try to get them back together.
Mice smell. Rabbits are similar to guinea pigs in terms of food requirements, but require a ton more space and exercise and destroy stuff and as a lady told me' if you don't have time for a dog, you don't have time for a rabbit'.
I am a 28 year old woman, but as I don't like free roaming animals in my home, i looked extensively into all the common caged pets...