Mathletics and IXL Maths never worked for us. I really believe in pen and paper at this age, especially if they need to master the basics.
At the beginning of year 6 my DD was getting low percentages in all the test papers that they did at school. My DD's school is outstanding, with SATs results much higher than national and also higher than the borough ones. DD was in the middle set for Maths, so I was surprised. I decided to work with her myself, to cover the gaps.
We used the GGP book series: SATs revision book standard level, together with SATs 10-min tests, again the standard level. She worked about 2 hours per week (2 sessions, 1 hr each) and I helped her with the areas where she was struggling. This was until Christmas.
In the Spring term I got her the SATs revision book advanced level, plus also the 10-min advanced level tests. Those took us almost to Easter break.
During Easter break and in the weeks just before SATs, I got her SATs practice papers - one standard, one advanced. She finished them all and required very little help from me. She was already getting high marks in these practice tests. This gave her a big confidence boost, she went to SATs with absolutely no stress.
In SATs she scored 114 in Maths and I couldn't be happier that all her hard work paid off and she was able to see the link between her hard work and results. In her final school report the teacher have her "above expectation" (5/5 rating) for the first time in primary school, and noted her great progress in year 6. The secondary she is going to will set based on Maths.
I would not leave it until year 7/8 if I were you... the more you leave it the harder it will be to cover the gaps.