Most definitely, the children can be involved. Some lovely ways include:
as ring bearers or in the wedding party
reading a poem (depending on the age)
scattering rose petals
carrying a chalkboard announcing the wedding party
giving (or revceiving) a gift or flower
combining coloured sand to represent the new family
As a couple you can incorporate a promise to the children as part of your own personal vows.
One last word...Children always do the best they can at a wedding. I have never yet seen a child muck up their special job. I've seen 18 month toddlers deliver the rings at the appropriate time, successfully.