I didn't take my 3 year old to my Mum's funeral. Many reasons but the main ones were that I didn't feel it was the right place for him and I wanted to grieve without worrying what he was doing. It was also the first time I left him with anybody other than DH or my parents! I have still only done it one other time.
Could you take them but have someone look after them during the service? Then they could come for the more informal part and meet your family?
It sounds like it's just me but I think funerals are very solemn events for young children and difficult for them to process.