Why do this problem?
is one on which learners will need to practise much addition and subtraction! It will require systematic work using an approach of trial and improvement.
What do the numbers from $1$ to $9$ add to? What do you still need to make $99$?
How about trying to make $90$ using the digits $1$ to $8$ and leaving the $9$ for adding on at the end?
Why not start by pushing the $1$ and $2$ together to make $12$?
Why not try pushing the $2$ and $3$ together to make $23$? What do you need now to make $90$?
Learners who find this straightforward could work out all the different numbers with less than four figures that can be made in the same way while still keeping the digits from $1$ to $9$ in order.
Children could use a calculator to work out what the numbers from $1$ to $9$ add to and the differences made by putting two adjacent numbers together to make a two-digit number.