Watch the Numberblocks episode 'Hide and Seek'.
Use some objects from your house to show the sums from the video. (Dry Pasta shells are really good for this)
4 + 1 =
Ask the children what the + means and remind them it is add/plus if they are not sure. When we add something on the number gets bigger. Do the first sum together and then see if the children can do the next ones themselves.
You could write them down for them to see or just read the sum (without answer) to them.
3 + 2 =
5 + 0 =
1 + 3 =
2 + 2 =
4 + 0 =
Repeat for taking away/subtraction.
Explain that when we take something away, the number we are left with gets smaller.
5 take away 1 =
5 minus 3 =
4 - 2 =
4 - 1 =
3 - 3 =
3 - 1 =
Give them time to see the plus/minus signs and look at how they are different.
(Use the different language - they haven't had much of an introduction to taking away so use the language of take away and minus and explain they mean the same thing.