I have done further research and it is usually recommended to get a tamarin rather than a marmoset because marmosets are more likely to get aggressive with age. They are both in the same species.
Jan - the marmosets in my area are legal if they are captive breed, but you do need to obtain a license. Marmosets are endangered species. The more research I do the more I realize monkeys are very moody and a poor choice for people with kids. I guess until Emilly is older - it is out of the question. Marmosets bite and love attention but refuse to be held - they are shoulder pets. They also live about 20 - 40 years and believe it or not - can die of loneliness. They are very susceptible to tuberculosis as well and need to be checked constantly.
I would really like to get a sugar glider - but they only live 2 years at best - and I also love hedgehogs - very sweet personalities and they love attention - but they are so darn pokey.
I put some links for pictures so you all can know what I am talking about.
http://www.geocities.com/Heartland/Fields/2662/marmosets.html
http://www.szgdocent.org/pp/p-tamgld.htm
http://www.pbs.org/wgbh/nova/vets/tamarin.html - this link is for the Cotton-Top Tamarin
(Saguinus oedipus)