It depends where you want to Marshal - some places no, some places yes.  My wife and I did some MGP practices at Union Mills and our girls (10 & 8) watched from behind the wall at the Church (well, I say watched - more like sat eating cakes and crisps from the Church stall).  However, at some of the other places we've been to there was nowhere close/secure enough for them to be.

Every extra marshal helps - especially the first practice session of the MGP - the last two years it was either cancelled completely or just a speed controlled newcomers lap because there weren't enough marshals...