A little preparation will help you defuse those expectations. Adult family parties can feel long and boring to children. Don't expect your children to warm immediately to people they haven't seen all year. Bending the normal rules a tad during this time can help your family cope with new experiences. Allow the 10-year-old to bring a toy or his favorite game—with one expectation. He is only allowed to play it as long as he includes one new family member each hour. Requiring him to include a family member even if it's only long enough to explain the game will help him create relationships that otherwise would be rejected. If playing with him is not an option, perhaps he can stop long enough to ask one person per hour what games they played when they were his age. He will begin to see adults as people he can talk with and they will see him as an inclusive child.
Allow the 7-year-old to wear play clothes (may be a good idea for all three) at first so she can go outside and play ball or do some other activity. Every hour she must go inside and share a snack with a family member. A small child bringing a cookie to Aunt Martha will help everyone's perception of your sweet child.
A tiny snack for the toddler won't ruin the day. Grandpa's prickly face, wrinkled skin and deep booming voice may be terrifying to a toddler. Bring tiny toys or snacks like bits of candy and distribute them to family members your toddler avoids. Gently encourage your toddler to play a game of hide and seek. Have the child look for the family member who has a special toy. Receiving a favorite toy from grandpa will facilitate understanding of the fact that grandpa isn't so scary. Encourage family members not to push relationships but to allow them to happen naturally.