I took my dds at that age on freebie tickets.I wouldn't have paid to go but we did have a good day (they didn't play up!) but we went with an older child and another adult which made the difference. really for this age group the playground and the face painting is the best for them and you can do that without paying to go in.
the tour itself is lots of talking and very interactive. theres only three bits which they would enjoy (probably) - a talk where the chairs shake, a dance floor that moves near the end and the chocolate bean ride (think its a small world from disney parks if you'vebeen) in the packing factory.
in the actual factory you can't take the buggy with you and there's lots of stairs and fair bit of walking.
we've just been again this year - dds now 6 and nearly4 and they had a fab time and keep asking me when they can go back!