K.B.
i think tha no matter how you ask someone will get offensive but also think about who you invite especially if they are family.. i think it is ok to go straight to the head of the family and let them know what is going on that you are saving for a bigger gift,.. most of the time the family(your brothers,sisters,parents,aunts,uncles,grandparents)are usually very accepting of gift ideas...
i have to usually give the gift ideas since my two childrens birthdays are both so close to christmas i have one the end of oct and the 2nd of jan so ideas are usually hard.. so asking to contribute to the larger gift is not horrible.
hope this advise helps
i am also a sahm for 4 years... of a 4 year old and a 2 year old.