Fish and Chip restaurant is reasonable, if you don't mind having chips. 20% Discount if you have annual pass, so makes it the cheapest place for hot drinks - which you can get to take-out.
The big restaurant now does pasta, pizza. The pasta is OK, not great by certainly edible. Would suggest adults get children portion, it is sufficient and includes a drink (milk/water/orange juice/apple juice), plus an apple.
I tend to take packed lunch though and get a hot drink. Today it was Hot Chocolate to keep us warm while watching Johnny Thunder (they did the shortened show version, due to water being so cold I expect). Regulars will know the differences between the various versions of the Johnny Thunder show
If buying tickets on the gate (or handing over vouchers) tell the member of admission staff that it is your son's birthday - and with luck he will get a badge.
Remember to take your winter woollies, it will be cold, especially come fireworks time. Suggest taking spare socks, so if a child (or an adult even) gets their socks wet, you have spares to change into - nothing worse than having cold wet feet.
When taking little one's (baby and say a 3 year old) I try to go with friends, so we have an adult who can stay with the babies. When that isn't possible, our rides are limited to: Fairy Tale Brook, Duplo Train, Big Train, Diggers (as I and baby can accompany 3-year-old). Don't tend to do anything else ride wise (have done boating school, but only when it's quiet, which it certainly isn't this time of year)... though do walk around mini-land, play in Duplo playground, go in the build & test. Today we did make-and-take (or whatever they were calling it) opposite 4D Theatre - where we made two pumpkins and a wizard and took them home (total cost £6).