I'm not religious so don't really know what the ceremony involves/what is expected, what sort of present would be expected for a joint 4 month old girl and 3 year old boys dedication? they are a friends children
no expectation at all, just your presence. Ceremony is all about thanking God for the gift of children, there is no water involved as there would be in an infant baptism (also Christening). Will probably be a short slot as part of the normal Sunday service.
A Children's Bible or Bible storybook would be an appropriate present, but may well be covered by any sponsors (Baptist equivalent of Godparents), so any small gift would be fine if you wanted to buy one. My DC had photo frames, moneyboxes, glasses, Peter Rabbit crockery, a child's wall clock, duplo and socks.