I was going to PM you but am actually interested to hear how other peeps approach their forms too. I am a parent of sn but no special form filling qualifications just lots of practice! You should look up the advice on the website to get a professional opinion but here is mine too fwiw.
I work on the basis that putting as much information on the form as possible is more likely to give DLA a realistic picture so fill in every possible bit no matter how vague it seems. I use up the 'anything else' bits with an essay on how long it takes to do stuff or how hard it is for dc to socialise etc.
Definately fill in mobility section anyway. I always add a bit of a rant about how unfair it is that most kids walk well by now and mine aren't. Emphasise the difficulties there are don't make light of any problems. I know your lo is walking but if tiredness/ clumsiness/ safety is an issue or you need a buggy or whatever then put that down on the form.
I would put from birth unless sn caused by later illness or accident.
You could put the school details then a note next to it clearly saying not attending school yet but due to on ... start date. Make sure you also say something along those lines in the something else bit just to make sure ie "child due to start school on ... not sure how well he is going to cope because of x y z".
You can put OT as "awaiting assesment" in the appointments due bit.
I always put how much therapy i do as well, so if kid sees physio once a month but you do physio for an hour every day then put an hour a day with a bit in brackets after saying by you etc. So put in portage daily, SALT daily. Most kids don't get the actual amount of therapy they need from a therapist, the teams just rely on parents to be doing the daily therapy and it is this info that DLA need. If you do any extra stuff like swimming or music classes then put that in to because strictly speaking it is therapy that you are paying for to help your child.
Use a black ballpoint and write neatly in capitals if you can. Add photocopies of any letters or reports you have that can back up your case. Write all of the form from the point of view of the worst case scenario, be blunt about how hard it is compared to other kids, don't miss out any difficulties or things that make your lives harder - does it take ages to get dressed, does child fall a lot, do you do lots of lifting and carrying, do you have to help with social interaction, are there dangers you have to look out for - all these things are much harder and time consuming with sn. No matter how small a thing it seems write it down.
PM if I can help more (may be tomorrow eve before I can reply though) and please please please photocopy the form before you send it away so you have it handy for next time and it won't be such a headache, also sometimes the forms get magically lost in the post.
Sometimes DLA will leave you alone for ages and other times you just filled out a form and another one gets sent out for re-assesment. I have a specific folder for all dla stuff so I can just go look up the old forms since most of it is just repeating the same stuff over and over. I also make a note of the date I sent the form and start phoning them to check they got it after about 5 weeks. So it does get easier the more you do it! It's also worth getting all the other forms done now for anything else you may be entitled to like carers allowance, income support, tax credit etc because although the DLA hasn't been approved yet the system can keep track of the application iykwim and there won't be a delay when it all comes through.
Hope this helps, sorry for the long post. [hconfused]