I've worshipped Shiva and Ganesh since childhood. Won't I offend them if I take up exclusive devotion to Krishna?
I don't think you have to worry. You can worship both Lord Ganesha and Lord Siva as assistants on the path of worshipping Lord Krishna! Lord Siva is the greatest Vaishnava and will assist in many ways if worshipped in that mood. He is happy to see His devotee seeking help in worshiping His Lord.
Also Srila Prabhupada said that Sri Ganesha can remove the obstacles on that path to bhakti and many Vaishnava temples in India have Sri Ganesh in the front of the temple, above the entrance, for this purpose. So it's not that you have to stop worshipping Lord Siva and Lord Ganesha but rather realize Their true position and seek their help, association and guidance in becoming a perfect devotee of Lord Krishna.
I hope this is helpful.