John Smeaton has posted on Why Comic Relief should be boycotted and I recommend reading his article. An important resource is the SPUC Charities Bulletin which can help you identify charities to avoid.
So which charities could you support in your almsgiving? If you need ideas, try thinking of different categories - mission, relief, pro-life, and local.
With charities that support the mission of the Church, such as the Association for the Propagation of the Faith, and Aid to the Church in Need, you are making it possible for the Church to enlarge its activity and therefore also its charitable work. A couple of years ago, Jackie Parkes had the idea of "Red Box Day" to support the APF. These mission charities also directly support the material welfare of the poor. A good example of a mission charity that is primarily engaged in development work is the Medical Missionaries of Mary.
Parish priests are always keen to support development work but many prefer for various reasons not to support CAFOD. (See this post, and this article.) A good alternative is to send the money to the Holy See's aid agency Cor Unum. The money can be sent to the Apostolic Nuncio with a covering letter advising that it is for Cor Unum. You will receive an acknowledgement from the Nuncio and, a few weeks later, a letter from Cor Unum.
In a country that kills so many of its unborn children, it is important to support pro-life work so you could send some money to your favourite pro-life organisation. Three excellent ones are SPUC, the Good Counsel Network and the Sisters of the Gospel of Life.
Local charities might include facilities for the homeless or the unemployed. We have some good ones in London - the Manna Centre, the Cardinal Hume Centre and the Simon Community. In Bexley Deanery, we have a "Third World Group" that organises donations for various small projects that are recommended by different parishes.
Catholic parishes and schools need never feel embarrassed for boycotting things like "Red Nose Day". Our parishes and schools are very generous to various charities and could choose some other light-hearted approach or activities to encourage youngsters if necessary. They don't need much encouraging - two children in my parish came to me today with the idea of doing a 10 mile sponsored cycle ride for the Manna Centre during Lent. That will be a great success, I'm sure.